when the cam 153 is in its neutral position and becomes the longest when the contact portion 154 of the cam 153 abuts against the stopper 156 according to this construction the switch 171 detects the neutral location of the cam 153 next description will be given of a mechanism for releasing residual pressure of the hst for traveling according to fig4 42 and 51 to 53 by depressing the clutch pedal 54 as mentioned above the arm of the hst for traveling 22 is set in its neutral position and the brake mechanism 74 is actuated after the crawler tractor is braked to some degree by engine brake of the neutral hst for traveling 22 at this time the hst for traveling 22 is stopped when one of a pair of passages which mutually connect a fixed type pump 22 a and a variable type pump 22 b in the hst for traveling 22 is hydraulically pressured higher than the other namely each plunger of the fixed type pump 22 a and the variable type pump 22 b is stressed at one end in its sliding direction by an oil pressure difference residual pressure between the above mentioned pair of passages so as to cause a.