To provide a portable machine of an electronic key system excellent in withdrawal operation of an emergency key.

The portable machine 1 of the electronic key system is provided with a rotation engagement member 30 which retains the emergency key 2 stored in a portable machine body so that it can be retained detachably. The rotation engagement member 30 has a rotating body 31 and a projection part 34. The rotating body 31 is supported with possible rotation while being energized on the surface side of the portable machine body by a coil spring 60. The projection 34, which can rotate with the rotating body 31, is provided so that it can be projected in the rotating direction in a position separate from the rotary axis of the rotating body 31. The emergency key 2 has an insertion hole 3 which is engaged with the projection 34 in a state where it is stored in the portable machine body. A push surface 51 of a key-fixation release button 50 is positioned on the surface of the portable machine body. The key-fixation release button 50 is constituted so that it can rotate the rotation engagement member 30 while moving it away from the surface of the portable machine body against the elastic force of the coil spring 60, and that it can withdraw the projection 34 from a through hole 3 by being pushed in the axial center of the rotating body 31.