A cooling fan control device, comprising: a hydraulic pump driven by an engine; a hydraulic motor that is driven with pressure oil discharged from the hydraulic pump and rotates a cooling fan used to supply cooling air to a heat exchanger (6,7); a direction switching valve that causes the hydraulic motor to rotate in a forward direction or in a reverse direction by switching direction of a flow of oil discharged from the hydraulic pump; a rotation direction selector switch for switching a rotation direction of the cooling fan; and a variable relief valve via which a discharge pressure at the hydraulic pump is controlled; characterized in further comprising a pressure sensor that detects the discharge pressure at the hydraulic pump; a control unit that, in response to an operation performed at the rotation direction selector switch to select a reverse rotation setting while the cooling fan is rotating forward, lowers a relief pressure setting at the variable relief valve to a predetermined lower limit value, and once the discharge pressure at the hydraulic pump detected by the pressure sensor is lowered to a predetermined switch-over pressure, executes control so as to switch the direction of flow of pressure oil to the hydraulic motor to the reverse direction by switching the direction switching valve and raises the relief pressure setting at the variable relief valve to a reverse rotation pressure setting, at which the hydraulic motor rotates in the reverse direction, over a first predetermined length of time. a hydraulic pump driven by an engine; a hydraulic motor that is driven with pressure oil discharged from the hydraulic pump and rotates a cooling fan used to supply cooling air to a heat exchanger (6,7); a direction switching valve that causes the hydraulic motor to rotate in a forward direction or in a reverse direction by switching direction of a flow of oil discharged from the hydraulic pump; a rotation direction selector switch for switching a rotation direction of the cooling fan; and a variable relief valve via which a discharge pressure at the hydraulic pump is controlled; characterized in further comprising a pressure sensor that detects the discharge pressure at the hydraulic pump; a control unit that, in response to an operation performed at the rotation direction selector switch to select a reverse rotation setting while the cooling fan is rotating forward, lowers a relief pressure setting at the variable relief valve to a predetermined lower limit value, and once the discharge pressure at the hydraulic pump detected by the pressure sensor is lowered to a predetermined switch-over pressure, executes control so as to switch the direction of flow of pressure oil to the hydraulic motor to the reverse direction by switching the direction switching valve and raises the relief pressure setting at the variable relief valve to a reverse rotation pressure setting, at which the hydraulic motor rotates in the reverse direction, over a first predetermined length of time. a control unit that, in response to an operation performed at the rotation direction selector switch to select a reverse rotation setting while the cooling fan is rotating forward, lowers a relief pressure setting at the variable relief valve to a predetermined lower limit value, and once the discharge pressure at the hydraulic pump detected by the pressure sensor is lowered to a predetermined switch-over pressure,