Otto Frank, 91, the father of Anne Frank, whose diaries convey a message of stunning hope in the face of the destruction of European Jewry during World War II, died Tuesday at a hospital in Basel, Switzerland.

The cause of death was not reported, but news agencies said he had experienced breathing difficulties for six months.

Mr. Frank was born in Frankfort, Germany. Anne, the second of his two daughters, was born there on June 12, 1929. In 1933, when Hitler came to power in Germany and began to put into practice his anit-Jewish policies, the family moved to Amsterdam, where Mr. Frank became a spice merchant.

In May, 1940, the Germans occupied the country. On Anne's 13th birthday, her father gave her a red-checked notebook and she began to keep a diary.

Two days later, the Frank family and four other Jews went into hiding in the attic of the 17th century building at No. 263 Prinsengracht. Their purpose was to escape the systematic elimination of the Jews of Europe which the Nazis had undertaken and which eventually claimed 6 million lives.

For 25 months, the Franks and their friends remained hidden in the loft, which Anne referred to in her diary as "The Secret Annex." When her notebook was filled, she jotted down her thoughts, as well and stories and poems, on whatever pieces of paper she could find.

The last entry is dated Aug. 1, 1944, the day "The Secret Annex" was betrayed to the Gestapo. The family was sent to various Nazi death camps. Anne and her older sister, Margot, both died in the Bergen-Belsen concentration camp in Germany. Anne was just short of her 16th birthday. Their mother also died. Only Otto Frank, who had been sent to the notorious Auschwitz camp in Poland, survived the war.

Anne's dairy also survived. Although the Nazis confiscated the furnishings and other belongings in the attic, they left the children's books. The pages of the diary later were picked up by Miep Gies and Elly van Wijk, who had been secretaries to Mr. Frank.

The girl described the strains of hiding, the family's hopes for liberation as Allied armies advanced across Europe in the spring and summer of 1944, the fear of discovery. What she wrote is notable not only as reportage but as an expression of hope and tranquility in the face of approaching disaster.

"It's really a wonder I haven't dropped all my ideals because they seem so absurd . . . Yet I can't build up my hopes on a foundation of confusion, misery and death," Anne wrote.

"I see the world gradually being turned into a wilderness. I hear the ever-approaching thunder which will destroy us too. I can fell the suffering of millions, and yet if I look up to the heavens I think that peace will return again . . .

"In spite of everything, I still believe that people are really good at heart."

Otto Frank returned to Amsterdam after the war and came into possession of Anne's manuscript.

"I never wanted the diary to be published and considered it as Anne's last will and testament," he said in an interview last year in connection with the television series "The Holocaust," a dramatization of the mass extermination of Jews. "But seven years later a friend convinced me that it should be published. Only information can insure that the horror and suffering of the past remains in the past and doesn't become the present."

"The Diary of Anne Frank" was translated into more than 50 languages and has sold more than 14 million copies. The royalties, and the proceeds from a movie and a play that were based on the book, were turned over to the Anne Frank Foundation in Amsterdam, the main task of which is to preserve the house where the family hid. The hiding place attracts about 300,000 visitors a year.

The 50th anniversary of Anne's birth was observed in The Netherlands and elsewhere in the world. In connection with that occasion, Otto Frank sent a message to the American Friends of the Anne Frank Center in New York City to thank them for tributes the group had helped to organize.

"This gathering is really in the spirit of Anne," Mr. Frank wrote. "We should not think of her, however, with pity or admiration only. Her diary should be a source of inspiration for all of us towards the realization of the ideals and hopes she expressed in it. Having seen herself a victim of anti-Semitism and Discrimination, she ardently hoped that after the war these evils would be abolished . . . I am sorry to say . . . that Anne's dreams have not yet come true."

Mr. Frank settled near Basel in the 1950s. At his direction, the manuscript of his daughter's great achievement is kept in the vault of a Basel bank.

Mr. Frank's survivors include his second wife, whom he married after the war.