The judge in the Iran-Contra case said Monday the major charges against former presidential aide Oliver L. North and three co-defendants boiled down to simple allegations of fraud.

"I am not trying anything else; it's a fraud trial," U.S. District Judge Gerhard A. Gesell told one of North's lawyers, John D. Cline, during a hearing on defense motions to dismiss the central conspiracy charge in the case."It is fraud in the sense that Chief Justice (William Howard) Taft and the Supreme Court thereafter have consistently applied" to the conspiracy statute, the judge said.

The defense, supported Friday by a Justice Department brief, contends North and his three co-defendants could not be prosecuted for obstructing Congress's foreign affairs authority by running a secret arms-supply network to the Nicarguan rebels.

The 1924 decision written by Taft held that the defendants did not have to violate a particular law in order to be guilty of obstructing government or depriving it of honest service.

Gesell also expressed puzzlement over defense contentions that the diversion of arms-sale profits to the rebels, known as Contras, did not violate the so-called Boland Amendment restrictions on military aid.

The defense contends that the series of amendments enacted by Congress to ban direct military aid by U.S. intelligence agencies did not apply to the National Security Council, where North was an aide until his dismissal two years ago.