PURPOSE: To enable anti-lock control to be effected by simple constitution by interposing an inlet valve and a pressure modulator in a part of a rear wheel braking circuit, and controlling the pressure modulator so that it can be switched between a communicating state and a blocking state according to the state of the anti-lock control.

CONSTITUTION: Front and rear wheel braking circuits I, II are connected to the outlet pressure chambers 18, 19 of a tandem main cylinder 14 driven via a braking force increasing device 16, and at the rear wheel braking circuit I, the wheel brakes 11, 12 of right and left rear wheels are connected to the output pressure chamber 18 via an inlet valve 37, a pressure modulator 21, and braking pressure modulating valves 51, 52. The pressure modulator 21, by controlling an ABS control valve 46 using an electronic control unit 61, takes a fundamental position where the connection of its primary chamber 32 is connected to the pressure outlet 38 of the main cylinder 14 during normal braking control; during anti-lock control, the primary chamber 35 is controlled to be displaced to an exciting position where it is blocked form the pressure outlet 38.