An anti-lock control device of a hydraulic brake system which enables a rapid building up of braking hydraulic pressure during normal braking but, once an anti-lock control is commenced, reduces the rise of the recovery of the braking hydraulic pressure so as to optimize the same for anti-lock control. The hydraulic braking system having a pneumatic pressure source, a brake valve, converting device for receiving the pneumatic pressure supplied from the pneumatic pressure source through the brake valve and converting the received pneumatic pressure into braking hydraulic pressure, wheel cylinders for receiving the braking hydraulic pressure, and pneumatic pressure control valves for controlling the pneumatic pressure in a pneumatic pressure chamber of the converting means so as to control the braking hydraulic pressure. The anti-lock control device has a comparator for comparing the pneumatic pressure in the pneumatic pressure chamber with the output pneumatic pressure of the brake valve, and an orifice valve which is put into a passage of pressurized air between the brake valve and the pneumatic pressure control valves when the comparator has judged that the pneumatic pressure in the pneumatic pressure chamber has come down below the output pneumatic pressure of the brake valve by a predetermined value. The orifice valve reduces the rate of rise of the pneumatic pressure in the pneumatic chamber so as to correspondingly reduce the rate of increase of the braking hydraulic pressure.