PURPOSE: To prevent locking from occurring in a fluid retarder device for a motor truck and the like by installing in series a switching valve and a control valve which increases pressure in an air chamber side in a pipe according to increase in sprung weight at the pipe which connects a pressure air source to an air chamber in an air/fluid converter.

CONSTITUTION: A switching valve 29 which selects a communication position with a retarder switch turned 312 on and a drainage position with the retarder switch 31 turned off is disposed at a pipe 30 which connects a pressure air source 4 with an air chamber 25d in an air/fluid converter 25. A control valve 28 is installed between a sprung part 32 and an unsprung part 33 of a vehicle, and pressure n the air chamber 25d side in the pipe 30 is controlled to increase according to an increase in sprung weight. With this constitution, it is thus possible to keep deceleration at a constant ratio regardless of a load.