HOUSEWIFE'S COMPANION, the Japanese magazine, to which Nancy Reagan gave a five- minute interview on her first full day in the White House, turns out to be not so--well--companionable after all. The magazine, it is now reported, presented a $1,000 token of its "gratitude" to White House National Security Adviser Richard V. Allen, who was involved in the interview. "Knowing this to be customary in Japan and not wishing to embarrass the Japanese journalist or the First Lady," the White House said Friday, "Mr. Allen received the honorarium and gave it to his secretary for safekeeping until he could ascertain the proper procedure for turning it over to the government." The money sat for eight months in a safe in the office Mr. Allen had briefly occupied before he moved into the West Wing, and was discovered in September, triggering an investigation that is still under way.

Well, fine. Whenever cash money in an envelope is passed in the White House, there darned well ought to be an investigation. Even if you accept Mr. Allen's explanation as the full and final version of this slightly bizarre affair, it seems as though he should have had the presence of mind--notwithstanding what no doubt was the hectic pace of Jan. 21, 1981--to comprehend the political dynamite inherent in that sort of transaction. His solicitude for the tender sensibilities of the Japanese might well have been matched by a regard for the proprieties of the Americans, not least his boss.

The investigation that is going on is being conducted in the Justice Department. Should it stop there? The ethics in government act of 1978, drafted in high post-Watergate dudgeon, would seem to have application here. Under its terms, the attorney general must investigate any allegation of illegal activity by a federal official and, if necessary, pass the probe on to a special prosecutor outside his department within 90 days.

We have questioned the wisdom of the way this special-prosecutor law has been applied in the past. The circumstances in this case, however, seem to be exactly of the sort that Congress had in mind when it established an investigative procedure outside the Justice Department. If nothing untoward has occurred, the public deserves to have the clearest possible assurance that this is the case. Mr. Allen, who insists the explanation of the affair is entirely innocent, surely can have no objection to letting a special prosecutor so demonstrate.