To provide an antenna structure of an electronic key in which it is difficult to change an antenna gain, even if a metal member which is removable from a key body is taken out of or inserted into the key body.

The electronic key 2 makes a vehicle execute an ID checkup by transmitting an ID code signal to the vehicle by air, an operation of a device set in the vehicle becomes possible based on a successful establishment of the ID checkup, and a mechanical key 25 is removable from a key body 20. An antenna structure of the electronic key 2 has a helical antenna 30 with an antenna element 31 wound like a loop, and a metal storage box 26 provided as a storage destination of the mechanical key 25 and lapped over the helical antenna 30. In the helical antenna 30, a resonant frequency is adjusted so that it can take a resonant frequency of a communication even if the metal storage box 26 is arranged in the vicinity thereof.