Mourir d'amour (1961)

Michel, a writer of detective novels, is married to Pat, a pretty woman he's been neglecting. One day, the results of a private investigation reveal that his wife is cheating on him. But he'd never contacted a private detective before. The facts are all the more disturbing in that they coincide with the detective story he is writing with his beautiful young secretary. Pat is murdered and found next to her dying lover, who is said to have attempted suicide. But the police soon realize that this is a double murder. The investigation develops; Michel is strongly suspected, as many presumptions seem to weigh heavily on him. The outcome of the investigation brings a surprise that clears Michel's name.