To provide a rotational mechanism for an armrest which is more easily fabricated and manufactured than heretofore in the configuration of regulating its rotation.

This rotational mechanism is movable at least between a usage position and a standby position with respect to a strut member with an armrest member attached thereto. The mechanism includes: a connection shaft member having a connection shaft to be the rotary axis of the armrest member; and a bearing member having a bearing part for receiving the connection shaft, which are respectively arranged at an armrest member side or a strut member side. The connection shaft member has a groove shape recession on the tip end surface of the connection shaft, which is arranged in a direction to be orthogonally crossed with the rotary axis. The bearing member has an opening edge part where the tip end surface of the connection shaft is exposed in a state where the bearing part receives the connection shaft of the connection member. The opening edge part comprises: a slide edge part positioned at a place being substantially equal to that of a groove shape recessing bottom part with respect to an axial direction; a regulating projection positioned at a place projected from the slide edge part; and, furthermore, a lock bar member which is fixed to the groove shape recession of the connection shaft member and has a length to be locked at the slide edge part with respect to a direction to be orthogonally crossed with the axial direction. The lock bar member is constituted to be rotated along the slide edge part in a fixed state to the groove shape recession, and also regulated in rotation by the regulating projection.

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