To provide a variable speed device for current control, where the amount of phase error has been reduced regardless of the difference in a speed change rate in the variable speed device by microcomputer control, where a speed control period has been extended, as compared with a current control period.

A speed control part 4 obtains a current command, based on the deviation between the speed setting value and the speed detection value of an electric motor for each speed control period, a current control part 6 obtains a voltage command based on the deviation between current command and the current detection value from a converter 5 for each current control period, whose period has been reduced as compared with the speed control period, and a voltage control part 7 controls the output voltage of an inverter main circuit 1 in accordance with the voltage command and the reference phase of the electric motor. The amount-of-speed-change detection part 8 counts the number of pulses from a rotary encoder 3A for each current control period, to detect the amount of speed change in the electric motor. A phase detection part 9 multiplies the amount of speed change for each current control period by a coefficient to obtain the amount of phase change, and a reference phase is detected by integrating the amount of phase change.