PURPOSE: To prevent breakage of a working machine with a hydraulic continuously variable transmission, by a method wherein, when a transmission gear is shifted through control of an auxiliary transmission gear, a relief valve, simultaneously, is opened and closed so that a traction force is normally kept at the same value.

CONSTITUTION: When an auxiliary gear shifting lever 2 is fallen in the direction of a high speed (a), a transmission gear 14 is shifted to the high speed side through an arm 4, rods 5 and 6, and a shifter 15, and high speed running takes place. Simultaneously, the one side 11 of an L-shaped lever 10 is lifted up, a relief valve 8 is pressed by the other side 12 for closing, and high torque is exerted on a motor to increase a traction force. Thereafter, with the auxiliary gear shifting lever fallen in the direction of a low speed (b), the transmission gear 14 is shifted to a low speed to perform running, and the relief valve 8 is opened through rotation of the L-shaped lever 10 to decrease a traction force. This constitution enables prevention of breakage of a working machine with a hydraulic continuously variable transmission even when a crawler is rotated powerfully during low speed running.