A portable device for electronically locking and unlocking a lock through wireless communication, the portable device comprising: a mechanical key for mechanically locking and unlocking the lock, the mechanical key including a first key cover on which an annular welding portion is formed, a second key cover on which a fitting portion is formed, and an internal space that is defined between the second key cover and the welding portion of the first key cover, the first key cover having an opening in communication with the internal space, and the fitting portion being arranged in the internal space; an engagement member arranged in the internal space, the engagement member being movable between a first position at which part of the engagement member projects from the opening and a second position at which the engagement member is substantially accommodated in the mechanical key; and a case including a key accommodation portion for removably accommodating the mechanical key, the mechanical key being physically separable from the key accommodation portion, the key accommodation portion including an engaged portion engageable with the engagement member that projects from the opening at the first position when the mechanical key is accommodated in the key accommodation portion, characterized in that the opening of the first key cover is formed by a tapered surface having an inner diameter that decreases from the inside toward the outside of the first key cover, the engagement member is conical and tubular and has a closed top, an open bottom end, and a tapered side surface, the angle of the tapered side surface of the engagement member being set to be substantially the same as the angle of the tapered surface defining the opening of the first key cover, the mechanical key is physically separable from the key accommodation portion by a linear pull-out motion which causes the engagement member to move from the first position, wherein the engagement member is engaged with the engaged portion, to the second position, wherein the engagement member is disengaged from the engaged portion, the mechanical key further comprises a coil spring, arranged in the internal space, for urging the engagement member toward the first position from the second position, wherein the coil spring includes: a first winding portion; and a second winding portion integrally formed with the first winding portion, which functions as a fixing portion adapted to be fixed to the fitting portion of the mechanical key, and having an inner diameter that is smaller than that of the first winding portion, wherein the second winding portion and the first winding portion are formed on substantially the same plane. a mechanical key for mechanically locking and unlocking the lock, the mechanical key including a first key cover on which an annular welding portion is formed, a second key cover on which a fitting portion is formed, and an internal space that is defined between the second key cover and the welding portion of the first key cover, the first key cover having an opening in communication with the internal space, and the fitting portion being arranged in the internal space; an engagement member arranged in the internal space, the engagement member being movable between a first position at which part of the engagement member projects from the opening and a second position at which the engagement member is substantially accommodated in the mechanical key; and a case including a key accommodation portion for removably accommodating the mechanical key, the mechanical key being physically separable from the key accommodation portion, the key accommodation portion including an engaged portion engageable with the engagement member that projects from the opening at the first position when the mechanical key is accommodated in the key accommodation portion, characterized in that the opening of the first key cover is formed by a tapered surface having an inner diameter that decreases from the inside toward the outside of the first key cover, the engagement member is conical and tubular and has a closed top, an open bottom end, and a tapered side surface, the angle of the tapered side surface of the engagement member being set to be substantially the same as the angle of the tapered surface defining the opening of the first key cover, the mechanical key is physically separable from the key accommodation portion by a linear pull-out motion which causes the engagement member to move from the first position, wherein the engagement member is engaged with the engaged portion, to the second position, wherein the engagement member is disengaged from the engaged portion, the mechanical key further comprises a coil spring, arranged in the internal space, for urging the engagement member toward the first position from the second position, wherein the coil spring includes: a first winding portion; and a second winding portion integrally formed with the first winding portion, which functions as a fixing portion adapted to be fixed to the fitting portion of the mechanical key, and having an inner diameter that is smaller than that of the first winding portion, wherein the second winding portion and the first winding portion are formed on substantially the same plane. a first winding portion; and a second winding portion integrally formed with the first winding portion, which functions as a fixing portion adapted to be fixed to the fitting portion of the mechanical key, and having an inner diameter that is smaller than that of the first winding portion, wherein the second winding portion and the first winding portion are formed on substantially the same plane.