To provide an armrest which can be movably installed on a seat back without complicating the constitution on the seat back side.

In this armrest 1 to be disposed on a side part of a seat back 3 of an automotive seat S, the armrest 1 is provided with an arm rest assembly 10 at least including an armrest part 10a, and a shaft member 13 connected to rotate a frame 11 of the armrest part 10a, and a holding means 20 to hold the arm rest assembly 10 on the seat back 3 side, and the holding means 20 is provide with a recessed part 21 in which at least a part of the shaft member 13 can be contained inside the seat back 3, where the shaft member 13 is held to be rotatable in the recessed part 21.