Abstract: In a portable information terminal system including a CPU and peripheral circuits, there is provided a discharge quantity table for storing remaining power levels. Each of the remaining power levels is defined based on a state of the portable information terminal system as a system state and a voltage of a power supply. A voltage detecting unit detects the power supply voltage in response to a voltage detection request. A control section monitors the system state, and issues the voltage detection request in response to a voltage check request. The control section refers to the discharge quantity table based on the power supply voltage detected by the voltage detecting unit and the monitored system state to determine the remaining power level of the power supply.