A variable gear ratio hydraulic power steering device 1 is provided with a variable ratio transmission mechanism 7 for varying the rotation transmission ratio between the rotational angle of a steering handle 31 and the rotational angle of a pinion shaft 33 and a flow volume control valve 40 for controlling the discharge flow volume of an oil pump 10 which circulates operating fluid. The variable ratio transmission mechanism 7 has a lock mechanism 75. A valve controlling controller 50 for controlling the flow volume control valve 40 switches main and secondary flow volume control maps for controlling the flow volume control valve 40, in dependence on the operational state of the lock mechanism 75. Thus, a failsafe function for the variable ratio transmission mechanism 7 is given to the variable gear ratio hydraulic power steering device 1.