To provide a control device for a synchronous motor which maximizes torque efficiency with arbitrary torque from maximum torque to torque zero.

Parallel translation coefficients for obtaining the quantity of parallel translation where a relation that, if either a first maximum torque efficiency curve which shows the relation between the rotational speed required for obtaining the maximum torque with maximum torque efficiency and an electromotive force phase difference angle or a second maximum torque efficiency curve which shows the relation between the rotational speed required for obtaining specified torque smaller than the maximum torque with maximum torque efficiency and the electromotive force phase difference angle is translated in parallel, it substantially overlaps the other, can be obtained, are predetermined in advance. An electromotive force phase difference angle generation means 9 decides the electromotive force phase difference angle required for generating torque ordered from a torque command at a specified rotational speed, from the phase angle at the specified rotational speed obtained from one maximum torque efficiency curve selected from the first and second maximum torque efficiency curves, the parallel translation coefficient, and the difference between the torque of one maximum torque efficiency curve and the torque ordered from the command torque.