PURPOSE: To obtain the stable deceleration effect for the temperature variation an the periphery, as for a hydraulic type revolution decelerating device which is used with a rotary shaft of a toilet cover or a toilet seat of a warm water cleaning device and controls the turning moment accompanied with the opening and closing of the toilet cover and toilet seat.

CONSTITUTION: A cylinder 3 which has a hydraulic chamber 2 which is filled with control oil and in which a rotary shaft 11 penetrates and a control wall 6 which radially projects from the rotary shaft 1 so as to divide the hydraulic chamber 2 into a pressurizing chamber 4 and a decompression chamber 5 in the revolution direction of the rotary shaft 1 are provided, and a control valve 9 is arranged so as to surround the control wall 6. A revolution decelerating device which is hardly given with the influence of the environmental temperature is obtained by constituting the control valve 9 from the material having the larger thermal expansion coefficient than the thermal expansion coefficient of the material used for the control wall 6 and the cylinder 3.