In a communication system in which data is transmitted and received in synchronization with a clock signal, a peripheral device cannot transfer data to a host device when the host device stops outputting the clock signal and thus the peripheral device cannot promptly transmit an interrupt request to the host device. A peripheral device transmits an interrupt request to a host device using a signal line for a clock signal when the clock signal output has been stopped. The host device receives the interrupt request, and resumes outputting a clock signal to enable data transmission and reception to and from the peripheral device. This enables the peripheral device to transmit an interrupt request to the host device promptly when the output of the clock signal from the host device has been stopped.