A motor control circuit calculates an α-axis current iα and a β-axis current iβ in a fixed coordinate system based on a W-phase (sensor phase) of an AC motor. The control circuit calculates, at each switching time point and each intermediate time point, the α-axis current iα from a current iw.sns sensed in the W-phase, and the β-axis current iβ from a differentiated value Δiα determined from the variation quantity of the α-axis current between every two successive switching time points or intermediate time points. Subsequently, the control circuit calculates a current phase xθ=tan−1(iβ/iα) relative to the W-phase. Subsequently, the control circuit calculates an estimation factor according to the current phase xθ and determines an estimated current iu.est in the U-phase of the AC motor by multiplying the sensed current iw.sns by the calculated estimation factor.