A hydraulic pressurizer system 30 comprises a drive source (an engine 2 and a motor generator 4 ), which drives a hybrid vehicle 1 , a mechanical oil pump 20 , which is driven by the drive source, an electrical motor 22 , which is activated by a 12V battery 24 , an electrical oil pump 21 , which is driven by the electrical motor 22 , and a control unit 15 , which activates the electrical motor 22. The pressurizer further comprises a ratio-change mechanism 7 , which is activated by hydraulic oil supplied by the mechanical oil pump 20 and by the electrical oil pump 21 to establish a speed change ratio, at which the rotational driving force of the drive source is transmitted to wheels 8 with a rotational speed change. For executing a control for stopping the drive source, the control unit 15 executes a cleaning operation, which operates the electrical oil pump 21 for a predetermined time before the stopping of the drive source.