To automatically brake a rotating body such as a wheel, roller, pulley when its rotational frequency increases.

A wheel 12 is supported so as to be rotatable around a sleeve 11, and around the wheel 12 a rubber tire is fitted, while an axle 10 is fixed to the sleeve 11 by a pin 15. The inside of the sheel 12 is made hollow, and therein a ring 16 is fitted, while a cover 17 is put thereonto. A cam 20 is formed integrally with the sleeve 11. Five rollers 21 are mounted around the cam 20 at equal intervals. A portion where the roller 21 is positioned is a protruding portion 20a while a portion between the rollers 21 is a recessed portion 20b. Ten holes 22 are formed radially in the ring 16, and in each hole 22 a piston 23 made round in its head is fitted. A groove 25 is formed on the outer circumferential surface of the ring 16, and the groove 25 forms a narrow passage 26 connecting the holes 22 to each other. Working oil 27 is filled in the holes 22 and the passages 26 so that the piston 23 in the hole 22 may be pushed out toward the cam 20.