To provide an electronic key with a charging function capable of securing user-friendliness by delaying such a situation that a battery runs out.

A battery voltage determination part 15 monitors a voltage Vk of a primary battery 6 as a main power supply for an electronic key 1. When a voltage Vx is equal to or less than a threshold value Vk, a restricting operation part 16 changes over the operation mode of an RFID tag 9 into an operation restricting mode to restrict the power consumption increasing operation of the RFID tag 9 during short-distance radio communication. A charge execution part 17 supplies part of power obtained by the operation restriction, out of radio waves received as a power supply from an external reader writer 8, to a power storage device 13 to charge the power storage device 13. Thus, the power storage device 13 can be used as an alternative power supply when the primary battery 6 runs out.