This invention relates to a braking device for a bicycle and in particular to one including an arm provided with a slot at an intermediate portion, a shoe having a rod at one side, a ball having a hole extending through a center thereof and a slit on its surface, a screw formed with a ring portion at one end and a threaded portion at another end, a knob having at one end a neck with a center through hole for receiving the rod of the shoe and at another end a slot with a curved surface at both sides, the neck of the knob being adapted to be received in the slot of the arm, and a nut engaged with the threaded portion of the screw.