To provide a position detector which can detect a cam position of a cam driving type control valve in a water softener in a short time.

The position detector 1 comprises a rotator 10 installed on the end of a rotary shaft 61 of a cam 60, a photo-interrupter 20, a hall element 30, and a control circuit 50. A notch plate 11 in which a plurality of notches 12 detected by the photo-interrupter 20 are formed circumferentially at regular intervals is installed on the outer end surface of the rotator 10. A permanent magnet 15 which is rotationally transferred accompanied by the rotation of the rotary shaft 61 and is detected by the hall element 30 is also installed on the outer end surface of the rotator 10. The control circuit 50 detects a rotational position of the cam 60 from the outputs of the photo-interrupter 20 and the hall element 30.