Fillon also said his wife's salary was "perfectly justified" and he would continue in the presidential race.
"I apologise to the French people," conservative candidate Fillon told a press conference, admitting it was an "error" that he regretted "deeply".
In an aggressive performance that saw him accuse the media of trying to destroy him politically, Fillon said his British-born wife Penelope's salary was "perfectly justified" and he would continue in the presidential race.
Addressing allegations that Penelope did not actually perform any duties to earn hundreds of thousands of euros, Fillon said: "No-one has the right to judge what a parliamentary assistant's job consists of, except the MP himself."
He said his wife had worked constantly to assist in his constituency business and that her average monthly salary of 3,700 euros ($4,000) after tax over 15 years had been fair for a woman with training in law.