A battery section of an electronic endoscope is provided with an EEPROM. The EEPROM stores charging time and the number of charging of batteries, necessary for calculating remaining power of the batteries, and customized information for assigning functions to first, second and third switches of an operation section. A remaining-power calculator calculates the remaining power of the batteries based on a relation between discharge voltage and discharge time of the batteries as well as on the charging time and the number of charging of the batteries read out from the EEPROM. A function assignment circuit assigns the functions to the first, second and third switches in accordance with the customized information read out from the EEPROM.