PURPOSE: To easily keep the speed of a moving body below the specific speed, so as to make the suitable constant speed possible by using dilatancy fluid as fluid making use of viscous resistance.

CONSTITUTION: The center part is formed by a plurality of struts, and right and left shaft parts are sealed into a housing 2 by O-rings 6 and bearings 5, so as to be rotatably fixed. Moreover a gear 4 is provided on the end of a rotor 1. There is a space 3 between the rotor 1 and the housing 2, and the distance is set to 0. 1 to 10mm. Dilatancy fluid 7 is filled in the housing 2 and sealed. The driving shaft is freely rotated without receiving little resistance in the case of the low speed. However solidification of the dilatancy fluid 7 is started when the revolutions is increased and then becomes the specific revolutions, thereby the rotor 1 suddenly receives high resistance, so as to be restrained from being rotated by the revolutions more than the specific revolutions.