A synchronous motor drive device that drives a plurality of synchronous motors connected in parallel to one another, comprising: a power converter configured to convert power from a power source to supply the power to the plurality of synchronous motors, a first switch device configured to electrically connect or disconnect the power converter and each of the plurality of synchronous motors or each of the plurality of synchronous motors except for one, and a controller that, based on a higher order command, is configured to perform stability judgment processing of the plurality of synchronous motors, determine the number of synchronous motors to be driven and a drive speed from among the plurality of synchronous motors, select at least one synchronous motor corresponding to the number of the synchronous motors to be driven from among the plurality of synchronous motors, control the first switch device corresponding to the selected at least one synchronous motor to electrically connect the power converter to the at least one synchronous motor and to supply the power, and instruct, to the power converter, the drive speed of the at least one synchronous motor to be driven, characterized in that the synchronous motor drive device further comprises a second switch device that electrically connects each phase of the synchronous motor electrically disconnected from the power converter by the first switch device, wherein, among the plurality of synchronous motors, one of the plurality of synchronous motors is directly connected to the power converter without installing the first switch device between the one synchronous motor and the power converter. a power converter configured to convert power from a power source to supply the power to the plurality of synchronous motors, a first switch device configured to electrically connect or disconnect the power converter and each of the plurality of synchronous motors or each of the plurality of synchronous motors except for one, and a controller that, based on a higher order command, is configured to perform stability judgment processing of the plurality of synchronous motors, determine the number of synchronous motors to be driven and a drive speed from among the plurality of synchronous motors, select at least one synchronous motor corresponding to the number of the synchronous motors to be driven from among the plurality of synchronous motors, control the first switch device corresponding to the selected at least one synchronous motor to electrically connect the power converter to the at least one synchronous motor and to supply the power, and instruct, to the power converter, the drive speed of the at least one synchronous motor to be driven, characterized in that the synchronous motor drive device further comprises a second switch device that electrically connects each phase of the synchronous motor electrically disconnected from the power converter by the first switch device, wherein, among the plurality of synchronous motors, one of the plurality of synchronous motors is directly connected to the power converter without installing the first switch device between the one synchronous motor and the power converter.