PURPOSE: To start an inverter and enable a motor to be started at the same time, by setting acceleration gradient set by an acceleration gradient regulation circuit till the motor is started, to the maximum degree.

CONSTITUTION: When the driving command signal IS of an inverter is applied, then a luminous diode PD is ignited by the comparator 20 of an acceleration gradient changeover circuit 2 till speed control voltage comes to the threshold value of 0.6V. Accordingly, a variable resistor 14 regulating acceleration gradient is short-circuited by a photo-transistor PT, and the output voltage of a Miller integration circuit 13 rises suddenly. At the moment of starting a motor 7, the output of the comparator 20 is inverted to a high degree, and the short circuit of the variable resistor 14 is released. Accordingly, the output voltage of the Miller integration circuit 13, namely, the rising gradient of speed control voltage is changed over to the gradient corresponding with the acceleration gradient of the motor set by the variable resistor 14.