The present invention is provided with a support mechanism for an armrest part that includes a gas spring. The support mechanism supports the armrest part in a state of being able to change a posture between an ordinary posture and a forward-tilt posture when the gas spring is locked, and the support mechanism supports the armrest part so as to make the armrest part move with forming an arc from a sitting corresponding position to a standing corresponding position that is located at a forward upper position when the gas spring is unlocked. A gas spring switch part that does not obstruct the locked state of the gas spring when the armrest part is in the ordinary posture and that releases the locked state of the gas spring at a time when the armrest part is in the forward-tilt posture is arranged between the gas spring and the armrest part.
