To provide a rotation auxiliary mechanism which functions to support a hinged door in an opened state.

There is provided a rotation auxiliary mechanism for a hinged door which includes a striker 13 provided at one of the hinged door and a frame body supporting the hinged door rotatably, and a catcher unit 30 which is provided at the other of the hinged door and frame body, and guides the striker 13 from a catch start position to a catch end position when the hinged door is closed so as to assist in closing the hinged door. The catcher unit 30 is provided with a support arm 35 which supports the hinged door in a state of opening at a preset angle, and the support arm 35 is supported by the catcher unit movably between a non-operation position where the support arm 35 does not overlap a relative movement trajectory of the striker 13 from the catch start position to the catch end position and an operation position where the support arm blocks the relative movement trajectory.