PURPOSE: To provide a fluid type retarder device with the excellent cooling capability of an operating fluid.

CONSTITUTION: A case 3 fixed on a vehicle body side member and filling an operating fluid to a retarder chamber 4 storing a rotor 6 and a stator 5 and a clutch device 13 capable of engaging or disengaging a rotary shaft 10 and the rotor 6 are provided, cooling fins 18 protruded outward are formed at an interval in the peripheral direction on the case 3, and a fluid passage 18a communicated to the retarder chamber 4 is formed along the cooling fins 18 in a fluid type retarder device The operating fluid flows into the fluid passage 18a and is satisfactorily cooled. The abnormal temperature rise of the operation fluid is prevented, and the viscosity reduction and transformation of the operating fluid can be avoided. The fluid passage 18a of the cooling fins 18 increases the quantity of the operating fluid, and the temperature rise of the operation fluid is suppressed.