PURPOSE: To improve safety by providing a means for judging whether the output signal from the button operating portion of a push-button type electronic speed change gear is a single push button signal or plural push button signals by a plurality of push buttons, on a CPU.

CONSTITUTION: A speed change button operating portion 3 has an advancing button, a backing button, a high/low speed changeover button, a neutral button, and a plurality of speed change buttons corresponding to each speed change gear. A system controlling CPU 12 judges whether the output signal from the button operating portion 3 is a signal from a single push button or signals from a plurality of push buttons and, at the time of a single push button signal, it is outputted to hydraulic valves 15, 16 as it is, to carry out the speed change of a main speed change gear 13 and a sub-speed change gear 14. On the other hand, not output is carried out at the time of a plurality of push button signals, but, when a neutral push-button signal is included, only the neutral signal is outputted. Even if a plurality of both high/low push buttons are simultaneously pushed by mistake, a command is always outputted to a low speed side operation. Thereby, the safety of the speed change operation of a tractor, etc. can be ensured.