An explosive device blew up in the passenger section of a Pan American World Airways jumbo jet carrying 285 people from Tokyo to Honolulu yesterday, killing a Japanese teen-ager and injuring four other people, authorities said.

Passengers screamed as the blast tore a hole in the cabin floor. Dense blue smoke filled the rear of the Boeing 747 jetliner, oxygen masks dropped from their compartments and the cabin lost pressure, but the pilot made a normal landing 20 minutes later at Honolulu International Airport.

John Leyden, a Federal Aviation Administration spokesman in Washington, said that as the plane was 140 miles from Honolulu the device blew up under the seat of a 16-year-old Japanese youth traveling with his parents. The youth was fatally wounded.

FBI spokesman Howie Dare said the bureau was investigating, but he and Leyden said it had not been determined whether the device was a bomb complete with trigger mechanism.

Leyden said the explosion blew a hole about 12 inches by 36 inches in the floor of the passenger cabin between the cabin and the cargo hold, but apparently caused no external structural damage.

"There was a big bang . . . . It was really strong. I saw a lot of blue smoke," said Zulfiqar Ali from Qatar, who was sitting with his wife toward the rear of the plane.

George Harvey of the FAA said that after the explosion the pilot quickly brought the plane from 26,000 feet to 10,000 feet because of loss of air pressure in the cabin.