The House ethics committee, saying it has found "reason to believe" that Rep. Charles C. Diggs Jr. (D-Mich.) misused office funds, decided yesterday to hold disciplinary hearings on 18 specific charges against Diggs.

The committee's action will, in effect, subject Diggs to a second "trial" on charges that he padded his office payroll and forced staff aides to pay him kickbacks. Diggs was convicted on 29 felony counts in a criminal trial last fall.

The congressman has appealed the conviction, and last week he asked the Committee on Standards of Official Conduct to delay any disciplinary action until the appeal has been decided. The committee rejected that request.

Diggs was sentences to three years in prison after his criminal trial. If he is found "guilty" in the disciplinary hearings before the ethics committee, he could be subjected to a fine, a reprimand, censure or expulsion from the House.

The House Administration Committee, meanwhile, said it has still not decided how to respond to Diggs' request that the House reimburse him for attorney's fees he incurs defending himself against the disciplinary action.

But the committee's senior Republican, Rep. William Dickinson (Ala.), said he would be "astonished" if the committee agreed to pay Diggs' defense costs. "I can't believe he asked us," Dickinson told a reporter. "But then, he's full of surprises.

Whether and how the House should punish Diggs has been a controversial issue among members since he was found guilty by a jury in federal court here.

In the trial, prosecutors charged that Diggs had illegally diverted more that $60,000 of his office funds by giving raises to his office employes and then demanding that they use the money to pay his personal bills. Diggs was also charged with using federal funds to pay employes of his family business in Detroit.

Immediately thereafter, Diggs relinquished his committee chairmanships and his right to vote for the remaining days of the 95th Congress. But after the November election, when Diggs was reelected by a large majority over token opposition in his Detroit district, he announced that he would take his seat and exercise his right to vote.

When the 96th Congress opened, junior Democrats and the Republican leadership called for Diggs to be expelled from the House, or at least to be denied a vote on the floor pending outcome of his appeal. The House rejected those proposals and referred the case to the ethics committee.

The committee hired a special counsel and spent four weeks looking into the charges against Diggs. Its action yesterday indicates that it found no significant evidence conflicting with the criminal jury's verdict.

Diggs will now have 21 days to respond to the committee's charges and the committee, under its rules, will hold a public hearing on the case "as soon as practicable" thereafter.

If the committee decides after hearings that Diggs did violate the House disciplinary rules, it can recommend punishment ranging from nothing to expulsion from Congress. The committee's recommendation will then be subject to a vote in the full House.

Diggs said yesterday that he was "disappointed" in the order for a disciplinary proceeding, and said he was concerned about the effect of "extensive publicity sure to be generated by the committee deliberations" on his pending appeal.

The only other instance that prompted the ethics unit to hold a formal disciplinary proceeding was the Korean influence-buying scandal that came to light during the 95th Congress. Four members were found guilty of rules violations in the Korean matter; they were eventually punished by a "reprimand," which had no impact on their privileges as members. CAPTION: Picture, REP. CHARLES C. DIGGS JR. . . . "disappointed" over action