PURPOSE: To presume a speed with high accuracy over the entire speed range by approximately calculating a primary crossing magnetic flux vector through a primary delay element, separately calculating the primary vector by using a presumed speed value, and so converging the presumed speed value to a real speed that both the calculated values become equal.

CONSTITUTION: A presumed primary crossing magnetic flux vector 1 is calculated by using a presumed rotating angle speed ωm and a primary current vector i1, and a presumed speed is so converged to a real speed as to erase its error. That is, the presumed vector 1 can be set to approximately presumed primary crossing magnetic flux vector 1' of the same level as an approximately primary crossing magnetic vector 1' by passing it through a primary delay/lead calculator 13 at this time, and an error due to the approximate calculation can be erased. Thus, the speed can be presumed accurately over the entire speed range.