The reception quality of the radio signal is detected in a wakeup period. The detected reception quality is compared with the first threshold. If the detected reception quality is equal to or higher than the first threshold, the duration of a wakeup period as a next reception target is set to the first time. If the detected reception quality is less than the first threshold, the duration of the wakeup period as the next reception target is set to the second time longer than the first time.