A synchronous motor drive device is a synchronous motor drive device that drives a plurality of synchronous motors connected in parallel to one another, and includes a power converter that converts power from a power source to supply the power to the plurality of synchronous motors, a first switch device that electrically connects or disconnects between the power converter and each of the plurality of synchronous motors, and a controller that, based on a higher order command, performs stability judgment processing of the plurality of synchronous motors, determines the number of synchronous motors to be driven and a drive speed from among the plurality of synchronous motors, selects a synchronous motor corresponding to the number of the synchronous motors from among the plurality of synchronous motors, controls the first switch device corresponding to the selected synchronous motor to electrically connect the power converter to the synchronous motor and to supply the power, and instructs, to the power converter, the drive speed of the synchronous motor to be driven.