Reclining mechanism

A reclining mechanism comprising:

• a ring-shaped first plate fixed to a seat back;
• a ring-shaped second plate capable of rotating with respect to the first plate and fixed to a seat cushion;
• a plurality of planetary gears in meshing engagement with internal gears provided in respective inner peripheries of the first plate and the second plate;
• a sun gear in meshing engagement with the planetary gears;
• a holding member rotatably holding the planetary gears and having an inner peripheral wall surrounding the sun gear;
• and
• a restriction member for restricting a rotation of the holding member with respect to the sun gear,
• wherein:
• the sun gear has an opposing wall section opposed to the inner peripheral wall of the holding member;
• the opposing wall section of the sun gear and the inner peripheral wall of the holding member define therebetween a wedge-shaped space in which the restriction member is displaceably disposed;
• and
• 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 with respect to a specific direction, where the specific direction is a 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, and the counter direction is a direction opposite to the specific direction.