PURPOSE: To shorten the time in a neutral state generated during the switching operation by constituting the title device so that the resistance sharply reduces and switching operation is carried out at a breath when the operating force exceeds a set value in the switching operation from a brake application state side to a reversal state side or in the reverse direction by a selecting mechanism.

CONSTITUTION: When an operating lever is operated to a right brake application position, a steering arm 48 is swung through a wire 53a and an arm 55, and the spool 36a of a variable relief valve 36 is pushed into a throttle opening degree closing side. Therefore, the brake power of a side brake is gradually increased, and a crawler traveling device is put into a grounded turning state. When the operating lever is operated to a right reversal position, the spool 35a of the second selector valve 35 is pushed in by the operating arm 48, and at this time, a resistance is generated in the pushing-in operation by the presence of a coil spring 57a. When a cylinder 57b projects from a groove 35b and rides over the outer surface of the spool 35a, the resistance due to the spring 57a sharply reduces, and the spool 35a is pushed in at a breath, and a super grounded turned state is established.