A mobile device is carried by a person to be authenticated. An acceleration sensor is installed in the mobile device and configured to output an acceleration signal corresponding to acceleration applied to the mobile device. A processing device is configured to execute determination processing for determining whether the person is walking based on the acceleration signal. A control device is configured to control operation of a controlled device based on a result of the determination processing and a result of processing for authenticating, by way of the mobile device, the person as a user of the controlled device. The processing device is configured to determine that the person is not walking in a case where an amount of change in the acceleration falls below a threshold before a prescribed time period elapses while the determination processing is executed.