To restrain turbulence of a flow of a hydraulic fluid by a cutoff plate when switched to a braking state from a nonbraking state in a fluid type retarder having a mechanism for inserting the cutoff plate for disturbing an air flow between both impellers at nonbraking time.

This fluid type retarder 2 brakes a rotary body by the hydraulic fluid, and has a rotor 24, a stator 25, a hydraulic fluid supply device, an air cutoff device 31 and an air pressure supply device. The rotor 24 is fixed to the rotary body. The stator 25 is oppositely arranged to the rotor 24. The hydraulic fluid supply device has a sump 22 for storing the hydraulic fluid. The air cutoff device 31 has the cutoff plate 32 taken in and out between both impellers 24 and 25, and can operate the cutoff plate 32 from the outside of both impellers 24 and 25. The air pressure supply device supplies air pressure to the sump 22, and supplies the hydraulic fluid between both impellers 24 and 25.