A PWM modulation unit modulates a three-phase voltage instruction, which is input, on the basis of a PWM method, and outputs a gate signal to each phase switching element of an inverter. A high-frequency component arithmetic unit calculates, at each time of switching of the inverter, a high-frequency component of a current occurring due to a voltage which is determined by the PWM modulation unit and is output from the inverter. An index arithmetic unit calculates, as an index R proportional to a rotational phase angle estimation error, from the high-frequency component of the current. A rotational phase angle estimation unit executes an estimation arithmetic operation of the rotational phase angle by using the index R. The invention provides a synchronous motor sensorless control apparatus which enables stable driving with simple adjustment, and does not cause an extreme increase in amount of arithmetic calculations.