PURPOSE: To stop a work machine and prevent a breakage by detecting the pressure of an adjuster cylinder performing the tension adjustment and cushioning of a caterpillar and setting the control valve of an oil hydraulic motor to a neutral position when this pressure is predetermined-valued or higher.

CONSTITUTION: When a forward pilot valve 15F is operated, the oil pressure from an oil hydraulic pump 12 is transmitted to the pilot chamber 14F of a control valve 14 via a shuttle valve 16, thus an oil hydraulic motor 13 is positively rotated and a work machine is advanced. When the caterpillar band of the work machine bites a foreign object and is abnormally strained during the travel, an adjuster cylinder 3 and an accumulator 11 exert a cushioning function. In this case, if the gas pressure of the accumulator 11 is low, the circuit pressure between the accumulator 11 and an operating oil chamber 10 becomes high. Then, this circuit pressure is guided into the pilot chamber 18P of a transfer valve 18, and if it is a preset pressure or higher, the transfer valve 18 is switched to the lower position. Thereby, the said valve 14 is switched to a neutral position.