The driver has a warning lamp (LED3) indicating that an output voltage of a battery (19) falls on or below a predetermined value. A switch switches-on and switches-off the warning lamp during a switched-on condition and a switched-off condition of another switch, respectively. The former switch is activated when the former switch is switched-on and the output voltage falls on or below the predetermined value, where the former switch is switched-off when a light source unit (12) is detached from an endoscope body (11) of a portable endoscope (10).
