To provide a portable device for an electronic key system reducible in manufacturing cost and miniaturizable.

The portable device 1 for the electronic key system capable of electrically locking/unlocking a specific lock by the collation of an identification code by radiocommunication comprises a groove-like recessed part 21 fitting therein an emergency key 100 capable of mechanically locking/unlocking the lock and allowing the fitted emergency key 100 to slide in a predetermined sliding direction (a); a locking part 25 formed in a body 10 and locking a thin-walled part 105 of a grip part 101 of the sliding emergency key 100; projecting parts 26, 27 fitted into clearance parts 28, 29 formed between the emergency key 100 and the recessed part 21 by the sliding of the emergency key 100 to prevent the sliding of the emergency key in a reverse direction to the sliding direction (a).