A hydraulic drive apparatus in which a hydraulic pressure motor is driven and rotated depending on an operation position entered from an operation lever. An oil pressure control circuit and method which detect pressures of operation oils capable of driving the hydraulic pressure motor to rotate and a pressure of an operation oil supplied from a hydraulic pump to a directional control valve. The oil pressure control circuit varies a set pressure of a main relief valve in a manner that feeds current to an electromagnetic relief valve and varies a set pressure of an electromagnetic relief valve. As a result, the oil pressure control circuit carries out such a control that a pressure of an operation oil received from the hydraulic pump is higher than a pressure of an operation oil for driving and rotating the hydraulic pressure motor by a predetermined pressure.