To solve the problem of an image scanner operated by USB bus power supply that a USB's specification current value is exceeded when a back light of an LCD monitor is turned on during scanning and the back light must be turned off during scanning.

The back light is turned on intermittently in synchronism with a driving signal of a motor. By turning off the back light for a period when much power is consumed by the motor, and by turning it on for a period when less power is consumed; the maximum value of a total power consumption of the image scanner is controlled and an excess of rated current of the USB is prevented.