PURPOSE: To prevent a power transmitting part from its damage due to wrong operation of switching a speed change mode by providing an electronic control unit which permits a changeover of a mode only when ratio of an output shaft rotational speed to an input shaft rotational speed is within a preset range by the changeover between the continued modes.

CONSTITUTION: When a value of a preset signal (m), output from a speed change mode changeover switch 7 set by manual operation and input to a judging circuit 9 is changed, a speed change mode switching actuator operation signal M corresponding to the value (m) is output, when a signal, output from a comparator circuit 11, is a mode changeover permitting signal (j). Reversely, when the signal, output from the comparator circuit 11 is a present mode fixed signal l, the speed change mode switching actuator operation signal M is not changed even when the preset signal (m) is changed. Thus even in the case of operating speed change mode wrongly, a power transmitting part in the inside of a transmission can be prevented from being damaged by actuating a wrong operation- preventing function by an electronic control unit.