To restrain power consumption in USB communication when the residual level of a battery gets low, so as to prolong an operable time for the whole equipment, in a USB device system coping with the "High-speed" of a USB standard operated by the battery.

In this USB device system 100 coping with the "High-speed" of the USB standard operated by the battery, a USB communication speed is changed from the "High-speed" to "Full-speed", and an operation clock frequency for a CPU 102 and various circuits is changed to a clock frequency for "Full-speed" communication, when the residual level of the battery is judged to come to a prescribed capacity or less and when the USB communication speed is the "High-speed".