To provide a spiral washer used in mounting a bracket on a wall board such as a plaster board, and a bracket mounting device capable of surely mounting the bracket.

This spiral washer is formed by spirally blanking a thin plate material having spring elasticity, and is successively screwed in an insertion hole penetrated through a wall plate mounted on a front face of a concrete wall face at constant interval while axially drawn and deformed to be fitted from the front surface side to the rear surface side of the wall board. The washer is held on a position of the insertion hole by a holding cord mounted on the spiral washer, a locking member of an anchor bolt provided with the rotatable landscape locking member on its tip part, is inserted from a hole of a mounting part of the bracket to a bolt insertion port of the spiral washer in the longitudinal direction along the bolt, then the locking member is laterally expanded, the spiral washer is elastically pressed to the rear face of the wall board by fastening the bolt, and the mounting part of the bracket is fixed to the surface side of the wall board.