PURPOSE: To allow a steering assist power to be selected according to operator's physical strength and his preference, by cutting off drive connection between an engine and an oil pump.

CONSTITUTION: When a vehicular speed increases to a value more than a set speed, an output signal from a comparator in a controller 27 is changed to H level, thereby selecting a selector valve 23 to its offset position. As a result, hydraulic oil in a hydraulic oil introducing chamber 22 of a variable pulley mechanism 18 is drained to minimize an effective diameter of a variable driven pulley 20 and render a tensile force of a belt member 21 zero, resulting in disappearance of a frictional force between the belt member 21 and both pulleys 19 and 20. Further, when a reference voltage generator 29 is manually operated to change its output signal, a changeover point from L level to H level of the output signal from the comparator of the controller 27 is varied, thereby allowing the vehicular speed to be varied when cutting a steering assist power.