PURPOSE: To prevent the generation of overload in a transmission even when starting is conducted at the neutral position of the transmission on a slope and a speed change lever is set at first speed position when car speed is accelerated in the automatic speed change gear of a bulldozer.

CONSTITUTION: The longitudinal inclination detector 5 of a car body is installed to an automatic speed change gear with a forward and backing lever 3, a speed- step select lever 4, a car speed detector 6 and a controller 7. When the speed- step select lever 4 is set to first speed position during a time when a bulldozer travels at speed or more determined at the neutral position of a transmission, the direction of rotation of the output shaft of the transmission is estimated from the inclination of the car body. A speed change step is engaged at third speed when the direction of rotation is the same as the direction selected by the speed-step select lever 4, and engaged at first speed when the direction of rotation is reverse, the difference of the number of revolution on the drive side and the driven side is reduced, and a shock at the time of engaging is diminished, thus preventing the generation of overload.