In an information storage device adapted to use a power supply supplied from an interface or a battery, a voltage input from the power supply is monitored in each time the power consumption of the device is gradually increased upon connection of the power supply, so that the power consumption or the function of the device is limited based on the result of monitoring. Alternatively, a voltage or a current input from the power supply during operation of the device is monitored, so that the power consumption or the function of the device is limited based on the result of monitoring.