To provide a work detector of a small size, a light weight and high reliability.

A detector is provided with a detecting rod 3 having a hexagonal shaft part 21 is fitted/supported to/by a hexagonal hole 13a of a case 1 side, which freely moves for detection and returns in the axial direction and of which the rotation is restricted in the peripheral direction, a magnet part 5, which is mounted on a detecting rod 3 side or a case 1 side and is movable with respect to the case 1 by moving the detection rod 3 in the axial direction for the case 1, and a switch part 7 which is supported by the case 1 and makes a moving contact ON or OFF based on the magnetic force of the magnet part 5 when the magnet part 5 is moved together with the detecting rod 3.