One of two variable displacement motors of a hydraulic traveling drive system (1) is an electric capacity control motor (3) which includes a capacity control electromagnetic proportional valve (32) which controls an inclined piston in a direction that the motor capacity becomes a minimum value in response to a shutoff of a control current supplied from a controller (Ct) which receives detection signals sent from pressure sensors (S5, S6) which detect respective pressures in a forward drive side main circuit (5) and a backward drive side main circuit (6) in a closed circuit, and the other variable displacement motor is a pressure capacity control motor (4) which includes a capacity control valve (43) which controls an inclined piston in a direction that the motor capacity becomes a motor capacity corresponding to pressures of the forward drive side main circuit (5) and the backward drive side main circuit (6) in response to the shutoff of the control current supplied from the controller (Ct).
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