According to one embodiment, a sensorless control apparatus for a synchronous motor, includes a PWM processing unit which pulse-width-modulates a three-phase voltage command, and thereby generates a gate command for an inverter, a high-frequency voltage calculator which obtains a high-frequency voltage component included in an output of the PWM processing unit or an equivalent output value, a high-frequency current calculator which obtains a high-frequency current component included in a current response value from a synchronous motor driven by the inverter, and an estimated angle calculator which calculates an estimated phase angle indicative of an estimated value of an angle of rotation of the synchronous motor, based on a plurality of pairs each including the high-frequency voltage component and the high-frequency current component which respectively include cosine components or sine components at an equal frequency, the pairs obtained for at least two different frequencies.