PURPOSE: To generate a required braking torque at the respective time points such as a braking start time, etc., and make working characteristics independent of a lapse of time by a method wherein the temperature of working fluid in a working fluid circulation path is measured and the flow rate of cooling liquid supplied by a heat exchanger is regulated to control the temperature of the working fluid.

CONSTITUTION: A retarder 10 has a line 51, an energy storage means 52, a heat exchanger 53 and a cooling circulation path including a line 54. When the temperature of working fluid is low, the working fluid which is not cooled is bypassed into a bypass line 55 in a working fluid circulation path by a continuously controllable a three-port two-position valve 50. After the temperature of the working fluid reaches a target temperature, all or a part of the working fluid is circulated through the cooling circulation path. The fluid temperature in the working fluid circulation path is measured by a highly responsive thermosenser, and held at most a maximum temperature by an evaluation electronic circuit.