There is provided a portable information terminal having a built-in radio communications device which minimizes a drop in the voltage of a battery and diminishes the operation threshold voltage of the portable information terminal, thereby lengthening the operatable time of the terminal. In the portable information terminal, when a radio communications section is in a transmission state, a microprocessor of an interface and a microprocessor of an information processing section are brought into a hold state, thereby deactivating the interface and the information processing section. As a result, the operating current required during transmission can be minimized. When the radio communications section is not in a transmission state, the information processing section and the interface are activated. Even when the radio communications section is in a voice communication state and when information is not input to the information processing section for a given period of time, the information processing section and the interface are deactivated. A voltage drop due to internal resistance of the battery is diminished by distributing or reducing the electric current dissipated by operation, in accordance with operating conditions. Consequently, the portable information device can be operated even when the voltage of the battery drops, thus lengthening the operatable time.