PURPOSE: To change speed to three speeds of high, medium, and low speeds by providing a speed switching valve supply-controlling pressure oil to inner and outer cylinder chambers corresponding to the pressure of a main pump supplying the pressure oil to a travel motor.

CONSTITUTION: When a first speed signal is inputted to a controller 56 with a first speed switch 591 operated, since an electric current is not sent to solenoids 551 and 552 and a switching valve 55 is placed to a neutral position N, a speed switching valve 42 is placed to a low speed position L and travel motors C and C rotate in a low speed and high toque. When a second speed switch 592 is operated, since the switching valve 55 is placed to a first position I with the electric current sent to the solenoid 551 and pressure oil is supplied to the pressure receiving chamber 47 of the valve 42 with the discharge pressure of a control pump P2 as pilot pressure, the speed switching valve 42 is placed to a neutral position M, and the travel motors C and C rotate in a medium speed and medium torque. When a third speed switch 593 is operated, since the valve 55 is placed to a second position II with the electric current sent to the solenoid 552 and pressure oil is supplied to the pressure receiving chamber 48 of the valve 42 with the discharge pressure oil of the pump P2 as pilot pressure oil, the speed switching valve 42 is placed to a high speed position H.