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.
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.
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.
"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."
