An impact absorbing device for a vehicle seat includes: a first base member; a second base member; and an impact absorbing member. The first base member rotatably supports the second base member. The impact absorbing member includes: a first fixing portion fixed to the first base member; a second fixing portion fixed to the second base member; and a deformable portion. The deformable portion is provided between the first fixing portion and the second fixing portion, and is configured to be bent by the rotation of the second base member. A distance between the first fixing portion and the second fixing portion in an undeformed state of the deformable portion has a value intermediate in a range of variation of the distance that varies depending on the bending of the deformable portion.