An electric rotary shaver including a cutter retaining plate equipped with cutter units each having an inner cutter and an outer cutter and detachably attached to an outer cutter frame of the shaver, and the shaver further including an attachment-detachment mechanism for mounting the cutter retaining plate to the outer cutter frame in a detachable fashion. The attachment-detachment mechanism includes a triangular boss and an engagement projection. The triangular boss is formed on the outer cutter frame and has engagement openings opened in its side surfaces, and the engagement projection is formed on a mounting knob held in cutter retaining plate. By turning the mounting knob with its engagement projection inserted in the boss and thus letting the engagement projection be engaged with the engagement opening, the cutter retaining plate is mounted to the outer cutter frame of the shaver in a detachable fashion.