In a hydraulic drive system, hydraulic oil discharged from an actuator pump is supplied to an actuator via an actuator drive circuit. In the hydraulic drive system, the hydraulic oil discharged from a fan pump is supplied to a cooling fan motor via a fan drive circuit, and the cooling fan motor rotates a cooling fan at a rotational speed corresponding to the flow rate of the hydraulic oil supplied to the cooling fan motor. A merge circuit connects the actuator drive circuit and the fan drive circuit to each other to cause the hydraulic oil flowing through the actuator drive circuit to merge into the hydraulic oil flowing through the fan drive circuit. When the merge condition is satisfied, the controller controls the merge circuit to connect the actuator drive circuit and the fan drive circuit to each other.