PURPOSE: To secure the performance of an engine brake and prevent the genera tion of engine-stall by judging that a braking force is applied or not to a vehicle with a judging means, and hindering the inhibition of the variable speed control when the applying of the brake is judged.

CONSTITUTION: When a brake device 107 is not actuated, a control means 101 carries out the automatic variable speed control on the basis of a term signal A from a sensor unit 105, and inhibits the change of the target gear position at the time of changing of speed or after the changing of speed. When the brake device 107 is actuated, the control means 101 judges the applying of a brake of a brake judging means 106, and releases an inhibiting function of the change of the target gear position. Thereby, the decision of the target gear position according to the content of the term signal A and the variable speed control based on this decision is constantly carried out. When the brake is applied quickly, the transmission is geared down so as to make efficient use of the engine brake and the engine-stall is also avoided.