A communication control processor controls communication of a portable communication terminal with external units, in which a data processing control processor performs communication with the communication control processor and controls a ROM, a RAM, an operating unit, and a display unit. The data processing control processor is supplied with a first clock signal, while the communication control processor is supplied with a second clock signal based on the first clock signal from the data processing control processor. Thus, each of the processors can transmit Universal Asynchronous Receiver Transmitter data even when the other processor is in a sleep state without knowing it, and the other processor can receive the data without an error in reception. Furthermore, the data processing control processor and the communication control processor can turn on/off a high-speed clock, as in a sleep control, independently of each other. Thus, sleep time can be maximized, and consequently power consumption can be reduced.