A brake includes an annular boot which can be expanded and contracted according to movements of a piston and is assembled to an opening part of a cylinder and to an annular groove on an outer peripheral face of the piston. The boot is assembled by: engaging one end of the boot with the opening part of the cylinder in a state where the piston is contained in the cylinder; moving the piston so that at least a part of the piston is withdrawn from the cylinder and that the other end of the boot is pressed by a pressing face of the piston; and fitting the other end of the boot into the annular groove of the piston, while a movable range of the other end of the boot is restricted by an expanding allowance of the boot whose one end is engaged with the opening part of the cylinder.