Provided is a hydraulic drive apparatus including a specific working actuator making a working arm perform a traveling assist motion upon the slip, first and second traveling motors, first and second hydraulic pumps, a traveling selector valve switchable to a neutral position and a straight traveling position, a communication throttle portion between a working fluid passage and a traveling fluid passage in the straight traveling position, and a switching control part switching the traveling selector valve to the neutral position when a single operation action is performed and to the straight traveling position when a specific combined operation action including a forward traveling operation and the specific working operation is performed. The switching control section adjusts the opening of the communication throttle section to decrease the flow rate therethrough with an increase in the difference between the first and second pump pressures.