A power transmission apparatus comprising: a battery; an electric motor configured to be drivingly rotated based on supply of electric power from the battery; an engine; a cell motor configured to start the engine; at least a hydraulic pump configured to be driven by the electric motor or the engine; a clutch configured to connect and disconnect motive power transmitted from the engine to the hydraulic pump; a switching means configured to permit or interrupt the supply of the electric power from the battery to the electric motor; an operating means configured to operate a working hydraulic actuator; an operating state detecting means configured to detect whether the operating means is operated; an engine revolution detecting means configured to detect the number of revolutions of the engine; a pump revolution detecting means configured to detect the number of revolutions of the hydraulic pump; and a pressure detecting means configured to detect discharging pressure of the hydraulic pump; a volume detecting means configured to detect displacement volume of the hydraulic pump; and a control device configured to carry out idling stop control in such a manner that, when it is detected that the operating means is not operated for a predetermined time or more, the engine is stopped, and the clutch is disconnected, and the supply of the electric power from the battery to the electric motor is interrupted, and when it is detected that the operating means is operated afterwards, the engine is started by the cell motor, and the supply of the electric power from the battery to the electric motor is permitted, and the clutch is connected after the number of revolutions of the engine reaches a predetermined value or more, and the supply of the electric power from the battery to the electric motor is interrupted, and characterized by that the control device calculates absorbing horsepower of the hydraulic pump, based on detection values by the pressure detecting means, the volume detecting means, and the pump revolution detecting means, wherein, with respect to the idling stop control, when it is detected that the operating means is operated, and the absorbing horsepower is less than a predetermined value, the control device permits the supply of the electric power from the battery to the electric motor without starting the engine, and wherein, when the absorbing horsepower is equal to or higher than a predetermined value, the control device allows the engine to start. a battery; an electric motor configured to be drivingly rotated based on supply of electric power from the battery; an engine; a cell motor configured to start the engine; at least a hydraulic pump configured to be driven by the electric motor or the engine; a clutch configured to connect and disconnect motive power transmitted from the engine to the hydraulic pump; a switching means configured to permit or interrupt the supply of the electric power from the battery to the electric motor; an operating means configured to operate a working hydraulic actuator; an operating state detecting means configured to detect whether the operating means is operated; an engine revolution detecting means configured to detect the number of revolutions of the engine; a pump revolution detecting means configured to detect the number of revolutions of the hydraulic pump; and a pressure detecting means configured to detect discharging pressure of the hydraulic pump; a volume detecting means configured to detect displacement volume of the hydraulic pump; and a control device configured to carry out idling stop control in such a manner that, when it is detected that the operating means is not operated for a predetermined time or more, the engine is stopped, and the clutch is disconnected, and the supply of the electric power from the battery to the electric motor is interrupted, and when it is detected that the operating means is operated afterwards, the engine is started by the cell motor, and the supply of the electric power from the battery to the electric motor is permitted, and the clutch is connected after the number of revolutions of the engine reaches a predetermined value or more, and the supply of the electric power from the battery to the electric motor is interrupted, and characterized by that the control device calculates absorbing horsepower of the hydraulic pump, based on detection values by the pressure detecting means, the volume detecting means, and the pump revolution detecting means, wherein, with respect to the idling stop control, when it is detected that the operating means is operated, and the absorbing horsepower is less than a predetermined value, the control device permits the supply of the electric power from the battery to the electric motor without starting the engine, and wherein, when the absorbing horsepower is equal to or higher than a predetermined value, the control device allows the engine to start.