PURPOSE: To make it possible to surely stop a vehicle when a speed changing means is returned, by providing a detecting means for releasing a brake in an initial manipulation subrange within the manipulation range of the speed changing means in association with the manipulation of the speed changing means.

CONSTITUTION: In a tractor in which rear wheels 10 are rotated by the output power of a hydraulic motor 16 through a gear type transmission 12 and a rear differential gear unit 13, a speed changing pedal 33 is composed of an advance pedal part 37 and a backward pedal part 38 which are depressed to certain degrees so that the discharge volume of hydraulic pump 15 is continuously and optionally controlled in an initial manipulation subrange B around the neutral position N within a manipulation range A. In this arrangement, there are provided detecting means 34, 35 including resilient contact elements 39, 40 which are arranged so that their front ends are located in the initial manipulation range B. Further, a control circuit 32 controls a solenoid valve 30 in accordance with outputs from the detecting means 34, 35, and accordingly, a normally effective brake 28 provided on the input side of the transmission 12, is released.