Israel's friendship with an increasingly outcast white-ruled South Africa is being strengthened this week with the first official visit of an Israeli Cabinet minister to this country.

Finance Minister Simcha Ehrlich, who arrived Sunday for an eight-day visit, is meeting top South African officials to discuss expansion of joint economic ventures in industry and trade, according to Israeli officials here.

But coming at a time when international hostility to the segregationist government here is perhaps at its height, Ehrlich's visit also signals Israel's intention not to desert what it considers a staunch and loyal friend.

"We are not blessed with an abundance of friends," said Israeli Ambassador Yitzak Unna. "Some friends, in moments of need, turn out to be fairweather friends and tend to leave us in the lurch. South Africa has never wavered in her attitude towards Israel," he said.

Ehrlich told a group of mostly Jewish businessmen on Monday that because of its duty-free policy toward industrial goods, Israel could offer South African investors an "attractive packet" for the export of industrial goods to both the European Economic Community and to the United States.

Israel could act as a "bridgehead" into the EEC and the United States for South Africa, Ehrlich told his audience. The Israeli government will soon create incentives to attract industrial investors, he added.

Yet no matter how attractive the incentives may be, there are few South African investors who could take advantage of them due to the scarcity of investment capital in South Africa itself. The country is more dependent on international credit and capital than ever before in recent years and faces a critical shortage of necessary investment capital.

Such joint industrial ventures, however, could be very valuable to South African exporters in the event of trade sanctions or boycotts against South Africa because of its racial policies.

They might also help reduce the trade imbalance between the two countries "which has rapidly been tipping to our detriment over the last three years," Unna said. This imbalance has been aggravated because as Israel has liberalized its import licenses, South Africa has tightened its licensing policy and foreign exchange restrictions. Correcting this imbalance is one of the main purposes of Ehrlich's visit.

Trade betwen the two countries has increased dramatically since 1973 when most black African nations broke ties with Israel as a result of October war.In 1976 trade between Israel and South Africa reached $90 million, according to South African figures.

Sixty percent of South Africa's exports to Israel are products of ISCOR, South Africa's state-owned iron and steel corporation. A large part of the rest consists of rough diamonds, and coal. Israel is the world's largest exporter of polished diamonds, and industry that accounts for 40 percent of its export earnings. Between 50 and 85 percent of the rough diamonds from South Africa.

In Israel, South Africa has participated in several ventures including construction of a railroad, a steel rolling mill and a hydro-electric plant.

Later in the week the Israeli delegation, which includes the secretaries for finance and trade, will sign an agreement to avoid double taxation on investments with South Africa. Ehrlich will meet with South African Finance Minister Owen Horwood and is expected to also hold talks with Prime Minister John Vorster and Foreign Minister Pik Botha.

His current visit was planned during Vorster's trip to Israel in April 1976, the first time a South African leader had gone to the Jewish state.

Ehrlich's visit is not meant to be "a tacit approval of separate development," Unna said, referring to South Africa's policy of strict racial separation. "We agree to disagree on those things. We have been thoroughly consistent in telling them we take a dim view of their internal policies," he said.