PURPOSE: To accurately calculate a current to a low speed by detecting the angle of a current vector, representing the type of the angle in a predetermined mode, and obtaining a digital value of the component on an arbitrary coordinates axis of the vector.

CONSTITUTION: An inverter pulse signal is supplied from a controller 30 to an inverter 28. A mode discriminating logic circuit 32 calculates from this inverter pulse signal a mode signal M1 corresponding to the type of the angle of a current vector. A function generator 34 is composed of a ROM in which a trigonometric function is written corresponding to the angle of each magnetic flux vector to calculate by inputting each mode signal M1 and the angle of the vector. Thus, the digital data of current components iM, iT on magnetic flux coordinates formed for the vector of unit amount are added to a D/A converter 36.