To provide a leaving sensor which can easily pull out a key pulled in any direction.

A leaving sensor 100 comprises a housing 1 that has an insertion port 1a, a key 2 that can be removably inserted to the insertion port 1a, a support body 3 that supports the housing 1, and detection means 4 that detects insertion and removal of the key 2. The housing 1 is pivotally supported by the support body 3 swingably. The support body 3 has a first support 5 that pivotally supports the housing 1 swingably with a first virtual straight line L1 in a first direction D1 as a center, and a second support 6 that pivotally supports the first support 5 swingably with a second virtual straight line L2 in a second direction D2 as a center. At a reference point P on a third virtual straight line L3 that passes through the insertion port 1a and is in an insertion and removal direction of the key 2, the first, second, and third virtual straight lines L1, L2, and L3 are orthogonal to each other.