When an UNREG voltage is equal to or less than a PREEND voltage, a notification for informing this state is sent from a lens-side CPU of a lens unit to a body-side CPU of a camera body by UART communication. At this time, the body-side CPU displays a warning, which informs that remaining battery is small, on an LCD. When the UNREG voltage is equal to or less than an END voltage, a notification for informing this state is sent to the body-side CPU 25 by the UART communication. At this time, the lens-side CPU waits until the body-side CPU turns off a power supply of the lens unit. The body-side CPU stops supplying electricity to the lens unit and displays a warning, which informs that the battery runs out, on the LCD. After that, the body-side CPU turns off a power supply of the camera body.