A mixer drum driving apparatus includes a liquid-pressure motor that drives the mixer drum to rotate, a liquid-pressure pump that is driven by a motive force of an engine and supplies working liquid to the liquid-pressure motor, an auxiliary liquid-pressure pump that is driven by an electric motor and supplies working liquid to the liquid-pressure motor, a pressure detector that detects a liquid pressure of working liquid discharged from the auxiliary liquid-pressure pump, and a switching valve that switches whether or not working liquid from the auxiliary liquid-pressure pump is supplied to the liquid-pressure motor. In a state in which the switching valve is closed and the mixer drum is being driven to rotate by the liquid-pressure motor that is driven by the liquid-pressure pump, the controller opens the switching valve if a liquid pressure detected by the pressure detector reaches or exceeds a preset liquid pressure.