Sen. Robert W. Kasten Jr.'s (R-Wis.) involvement in some questionable Milwaukee real estate deals has resulted in his being named in a civil complaint, and could cost him more than a half million dollars.

Kasten and nine others were named in a suit filed April 13 by the Wisconsin securities commissioner alleging that they took part in fraud and a number of securities law violations, including sale of unregistered securities and sale of securities by unlicensed agents.

The suit contends that Kasten and the others, all partners in various Milwaukee real estate deals, were involved in sales of up to $50 million in securities to thousands of investors. Some of the 30 related businesses named have failed, leaving hundreds of investors with little chance of recovering their money.

Kasten says his involvement in the partnerships was minimal, and investigators say it is unlikely that the senator will face criminal charges. However, the Wisconsin Justice Department has entered the case, and charges could be filed against others.

The main target is Oliver Plunkett, the Milwaukee promoter who organized and headed the shaky financial empire.

Plunkett filed for reorganization in federal bankrupcy court last month, putting further proceedings against him and his partners on hold. A judge then issued a temporary restraining order barring the sale of any properties.

As a general partner with Plunkett in four real estate ventures, Kasten can be held liable for part of the businesses' debts. Kasten resigned from one of the partnerships shortly before entering the Senate in January, 1981, but did not resign from three others until after the complaint was filed.

Attorneys say the resignation will not shield him from liability, civil or criminal, for actions taken while he was a partner.

This is not Kasten's first controversy in his home state over finances.

Last year it was disclosed that he had failed to pay his state income tax in 1977, while serving the second of his two terms in the House. Kasten at first claimed that he had paid the taxes, then retreated from that position when he was unable to produce a canceled check.

He admitted a month later that he had made a mistake, made his belated payment of $2,269 plus an undisclosed amount in interest and penalties, and said he hoped the "embarrassing" incident was closed.

Kasten, 39, comes from a prosperous banking and manufacturing family. He became involved with Plunkett in 1979, after leaving the House and following an unsuccessful run for governor.

In addition to the partnerships with Plunkett, he became vice president of a company owned by Plunkett, but said his involvement in it declined after he announced his candidacy for the Senate early in 1980. He upset Democratic Sen. Gaylord Nelson in the 1980 election.

One of the Kasten-Plunkett partnerships, East-Town Properties, folded late last year, leaving $3 million owed on its property, $1.7 million in other debts and several hundred unhappy investors, including Madison Mayor Joel Skornicka and his wife, Carol.

Among the charges against Kasten, Plunkett and four others are that they allowed illegal loans to be made by their other real estate partnerships to the floudering East-Town venture. That, Securities Commissioner Richard Malmgren said in his complaint, was a direct violation of an earlier order he had issued and a violation of the fraud section of state statutes.

East-Town still owes $420,000 to four other Plunkett partnerships, including two in which Kasten was a general partner.

The full extent of Kasten's liability has not been determined and probably will not be for some time. The main question remaining is whether the downfall of East-Town and some of the other failed ventures will drag the rest of the Plunkett-Kasten partnership down with them.

At a news conference last month in Milwaukee, Kasten denied that he has ever broken any securities laws, and said, "My participation and my part of this complaint is very, very limited." He also denied having any knowledge of any illegal loans.

Kasten promised to make a full financial disclosure of his role in the partnerships. Since then, however, he has turned down requests for interviews on the subject. His aide, Kim Gigstead, said that the senator is "concerned about creditors and his financial obligations and is continuing to investigate."