PURPOSE: To suppress abrupt fluctuation of braking force and improve durability in a hydraulic rotary deceleration device which controls rotational force generated by opening and closing of a cover and a seat of a hot water serving toilet.

CONSTITUTION: A cylinder 3 has an inner space divided by a partition wall 1 projected inward, and a plurality of hydraulic chambers 2 in which control oil is charged. A rotary shaft is coaxially arranged in the cylinder 3 such that an outer peripheral surface is in a sliding condition with an end of the partition wall 1, it surrounds a control wall 10 projected so as to divide each of the hydraulic chambers 2 into a pressurizing chamber 8 and a pressure reducing chamber, and the outer peripheral surface of a control valve 13 having a substantially U-shaped sectional area is in contact with the cylinder 3. A bypass passage 14 is formed on an outer peripheral surface of the rotary shaft 4, which passage 14 extends from a lower end of the control wall 10, has an end communicated with the adjacent hydraulic chamber 2 through the sliding part between the end of the partition wall 1 and the rotary shaft 4 from starting braking operation to the rotation by a specified angle θ, and has a sectional area which is gradually decreased.