To provide a washer which can be inserted sideways into a bolt or the like without falling off when loosely fastened, and to provide its inserting and mounting method.

The washer with an open/closed piece comprises the open/closed pieces 4, 4 formed with slit grooves 3, 3 which are vertically cut from an insertion hole 2 of the washer 1 to an outer diameter portion at a slit width W1 and cut from the insertion hole 2 to the right and left cross directions at slit widths W2, W2 without passing through the outer diameter portion and whose bottom portion is bendable from the cut extending end along a line vertically cut at the slit width W1. The washer is inserted and mounted sideways into the bolt or the like with the open/closed pieces 4, 4 being open/closed.