PURPOSE: To smoothly switch the operating state of an inverter even when the carrier frequency difference between a DC braking state of a motor by means of a high-carrier frequency and an inverter operating state by means of a low-carrier frequency is large by providing an inverter operation stopping period between both states when the DC braking state is switched to the inverter operating state.

CONSTITUTION: The period before time t1 is a high-carrier frequency domain corresponding to the DC braking state of a motor and, during the period after time t2, the state is switched to a low carrier frequency domain corresponding to an inverter operating state. Then an inverter operation stopping period of duration Td is provided between the time t1 and t2. An electric current Ii does not exceed its set upper limit value Iip, but quickly decreases to zero, and re-outputted at a designated normal value after time t2, and thus, the DC braking state is smoothly switched to the inverter operating state.