PURPOSE: To facilitate the speed change operation of a continuously variable transmission by a hydraulic pump so as to enable positive speed change to a specified speed.

CONSTITUTION: A hydraulic pump 4 formed of a rotor 17 and a casing 12 having at least a pair of through holes 21, 22 for intake and discharge into a housing 1 is provided rotatably. A first rotary shaft 23 is connected to the rotor 17 of the hydraulic pump 4, and a second rotary shaft 25 is connected to the casing 12. A first oil pressure chamber 5 partitioned by the casing 12 of the hydraulic pump 4 and communicated with one through hole 21, and a second oil pressure chamber 6 communicated with the other through hole 22 are provided in the housing 1. Both oil pressure chambers 5, 6 are connected to each other by a hydraulic circuit 10 with a control valve 9 interposed.