The Justice Department has been conducting a preliminary inquiry into the financing of advertising for the 1976 Carter presidential campaign by the Gerald Rafshoon Advertising Agency.

The inquiry was not prompted by any allegations of criminal wrong doing but by two reporters who brouhgt "information" to the attention of the Justice Department's Criminal Division five months ago, according to Justice Department spokesmen Terry Adamson.

Adamson said the department's "sensitivity" to the special prosecutor requirements of the new ethics in government act induced officials to start looking into the Rafshoon matter to determine whether a special prosecutor was necessary.

Adamson described the process under way since January as an "expedited inquiry." But Rafshoon, who is now President Carter's chief media adviser, said that he had not been questioned by the department and that to his knowledge no one else connected with the agency had been questioned. Nor have any records been subpoenaed, he said. In fact, Rafshoon said he was unaware of any Justice Department activity concerning the agency, which was sold for him last month by his trustee, William Stack Jr. of Atlanta.

Adamson would not identify the two reporters who were said to have approached Philip Heymann, chief of the Department's Criminal Division, and triggered the inquiry by telling him the department "should know about" the Rafshoon matter.

News media investigations and articles since January, including a recent article in The Nation magazine, have focused on the extension of credit to the Carter campaign by the Rafshoon agency.

According to Carter campaign finance reports, the Rafshoon agency extended $645,000 in credit to the Carter primary campaign, helping it to buy advertising at a time when a cash crunch had set in because of a cutoff of federal matching funds.

Newspaper and magazine reports have questioned how the Rafshoon agency could have afforded such generosity and whether it was assisted by funds from Bert Lance's National Bank of Georgia or the Carter family peanut business.

No evidence of such assistance has been reported, and according to Justice Department sources, the two unidentified reporters did not furnish it.

One special prosecutor, Paul Curran, is already investigating the relationship between the campaign, the peanut business and Lance's bank. Asked yesterday whether he would also look into the Rafshoon matter, Curran said the Justice Department was already doing so.

Adamson said that if the current inquiry produces any criminal allegations, the matter would be handled under the special prosecutor law. This requires a special panel of judges either to appoint or decline to appoint a special prosecutor for probes of top White House officials or the president. CAPTION: Picture, GERALD RAFSHOON