Reclining mechanism

Disclosed is a reclining mechanism comprising: a first plate (1) fixed to a seat back; a second plate (2) fixed to a seat cushion; a plurality of planetary gears (3) in meshing engagement with internal gears of the first and second plates; a sun gear (4) in meshing engagement with the planetary gears; and a holding member (5) rotatably holding the planetary gears. An inner peripheral wall of the holding member and an opposing wall section of the sun gear define therebetween a wedge-shaped space in which a restriction member (7) is displaceably disposed. The opposing wall section of the sun gear is formed such that a width of the wedge-shaped space is gradually narrowed toward a counter direction opposite to a specific direction along which the first plate is urged to be rotated according to a force applied to the seat back in a rearward tilting direction.