Ruffier netted into his own goal after a Mathieu Bodmer header hit the post and bounced off the keeper's hand into the net in the 32nd minute to earn PSG a narrow win that gave them 40 points from 19 games.
They lead Montpellier by three points after the southerners slumped to a 4-2 defeat at Evian Thonon Gaillard having conceded three goals in eight minutes.
Champions Lille were four points off the pace in third place after conceding a last-gasp equaliser in a 4-4 home draw against Nice.
Fourth-placed Olympique Lyon, on 35 points, faltered with a 1-0 defeat at Valenciennes.
PSG looked weary at Saint-Etienne, who had won four of their last five games, and the winter break could not have come sooner for the whole squad although there will be no rest for the club's officials as a busy transfer market looms.
Former England international David Beckham has been tipped to join PSG by the French media, while unsettled Manchester City striker Carlos Tevez is also a target of the Qatar-owned club, who have already spent over 80 million euros last summer.
Montpellier have only picked up one point from their last three games although they went 2-1 ahead nine minutes into the second half thanks Olivier Giroud's 13th goal of the season.
They then conceded three between the 68th and the 76th minutes as their defence sprung a leak. Saber Khelifa, Cedric Cambon and Brice Djadjedje were all on target to swing the match Evian's way.
Lille were 3-2 down when Eric Mouloungui was sent off in the 69th minute for a second bookable offence but piled the pressure on Nice with Belgian prodigy Eden Hazard equalising with a fine curled shot before Florent Balmont chested the ball forward and volleyed into the top corner two minutes from time.
Nice defender Francois Clerc, however, earned his side a deserved point when he netted from inside a packed goalmouth four minutes into stoppage time.
"We had a very tough first half and then we went ahead with two superb goals so we are disappointed," Hazard told French TV channel Foot Plus.
"But it was a great game for the fans. We'll rest now and come back stronger after the break."
Lyon, too, lacked energy at Valenciennes, losing their first game in more than a month after defender Aly Cissokho scored an own goal midway through the first half.
Bottom club AC Ajaccio improved their outlook with a 2-0 win at fellow strugglers Sochaux.
