A processing device comprises an interface and its control circuit for performing data transfer in synchronization with an external clock, an internal oscillator, and an interface and its control circuit for performing data transfer by using an internal clock generated by the internal oscillator. In the processing device, a clock control circuit that switches a system clock between the internal clock and the external clock in accordance with the interface is provided. When the system clock is switched, the switching is performed after the CPU is set in a sleep state, and after the switching is completed, the sleep state of the CPU is released to restart the operation.