1. A motor control device comprising: an inverter which drives an electric motor; a vector controller which converts a detection current of the motor on synchronous coordinate axes synchronized with an output frequency and estimates a rotating speed of a rotor of the motor on the synchronous coordinate axes, thereby determining an output frequency and further determining a command output voltage so that a current command value and a current detection value correspond with each other, the vector controller converting the command output voltage to that on coordinate axes at rest and supplying the converted command output voltage to the inverter to drive the inverter; a speed control unit which determines a torque command value so that a speed command value and a speed estimate value estimated by the vector control unit correspond with each other; a current command calculation unit which calculates the current command value based on the torque command value and supplies the current command value to the vector controller; and an inertia moment estimation unit which obtains a load torque estimate value of a load apparatus connected to the motor rotor and estimates an inertia moment that is an addition of an inertia moment of the motor rotor and an inertia moment of the load apparatus, based on a deviation between an acceleration/deceleration torque output value obtained by subtraction of the load torque estimate value from a value of torque generated by the motor and an acceleration/deceleration torque estimate value obtained by multiplying an amount of variation of the speed estimate value per unit time by an inertia moment estimate value, in a state where the speed command value is varied by a predetermined speed width in a first one of directions of increase-decrease while the motor is driving by a speed command value not more than a rated rotational speed and the rotational speed of the motor, and the speed command value is varied by the predetermined speed width in a second direction opposite the first direction after lapse of a time for which the rotational speed of the motor settles, and after lapse of a time for which the rotational speed of the motor settles, variation of the speed command value is repeated so that the speed command value is re-varied in the first direction, wherein the speed control unit is configured to be capable of adjusting a control parameter based on the inertia moment estimate value estimated by the inertia moment estimation unit.