To provide a high-quality sliding position detector which maintains excellent position detection over a long period of time.

This detector is provided with a case 1, an operating shaft 9 which protrudes from the case 1 in a freely slidable way, a permanent magnet which moves being interlocked with the operating shaft 9, and a magnetism detecting element 12 for detecting the magnetic field of the permanent magnet 10. The inside of the case 1 is divided into a first housing chamber 2 for housing the operating shaft 9 and the permanent magnet 10, and a second housing chamber 3 for housing the detecting element 12 by a partition 1a stretching in the direction of sliding of the operating shaft 9. A first opening 2a is formed on one end side in the direction of sliding of the first chamber 2, and a second opening 3a is formed on the other end side in the direction of sliding of the second chamber 3. The first and the second openings 2a, 3a are closed with a first lid 4 and a second lid 5 respectively.