PURPOSE: To improve the operating feeling of a vehicle and prevent the rear-end collision by switching a decompression means from the preset pressure condition at the traveling speed which exceeds the specified value to the higher pressure condition when the signal is detected by a traveling speed detecting means, and supplying the high pressure air from a pressed air source to an air-liquid converting device.

CONSTITUTION: When a traveling speed detecting means 100 is turned ON, a third switching valve 95 is set to the high pressure position, while a second switching valve is set to the intermediate/high pressure position. Thus, the high pressure air is supplied to an air chamber 25d of an air-liquid converting device 25 through a low speed decompressed valve 93. As a result, when the vehicle is traveling at the speed lower than the specified value, the higher pressure air whose pressure is higher than the set pressure at the traveling speed exceeding the specified value is supplied to the air chamber 25d, and further large braking torque according to this high pressure air is generated to a fluid type retarder 11. This arrangement complements the shortage of the braking torque of the fluid type retarder 11 at the traveling speed lower than the specified value, and an appropriate deceleration is obtained.