A controller that controls an electric compressor, which is mounted to a vehicle and configures a two-stage compression refrigeration cycle device, has a deceleration section, an operation stop section, and a restart section. The deceleration section reduces a rotational speed of an electric motor by controlling an AC current which is output to the electric motor, when a two-stage compression mode, in which an intermediate-pressure refrigerant flows into the electric compressor from an intermediate-pressure port, is performed and an operation stop request to stop the electric compressor is made. The operation stop section stops the electric motor after the deceleration section reduces the rotational speed of the electric motor. The restart section restarts the electric compressor when the operation stop request is canceled after the operation stop section stops the electric compressor.