A rotational speed pickup 71 (traveling speed detection means), a hydraulic pressure sensor 72 (traveling operation detection means), a hydraulic pressure sensor 73 (drive status detection means), and a voltage sensor 74 (transmission shift position detection means) detect parameters regarding the traveling state of a hydraulic traveling vehicle. A control unit 80, a solenoid proportional valve 81, an engine control system 82, and a motor regulator 33 determine the operating state of the hydraulic traveling vehicle based on the parameters regarding the traveling state, and control the equivalent displacement of the traveling system including a hydraulic traveling motor 14 and the maximum flow rate supplied to the hydraulic traveling motor 14 according to the result of determination. The pump flow rate and motor displacement are controlled according to the traveling state in this way, thereby ensuring favorable traveling performance without degrading the fuel efficiency.