To provide a turn position detecting device for an erector capable of continuously detecting and calculating the turning direction and turn angle of a turning ring on a real-time base.

The turn position detecting device is used for the erector provided with the turning ring 2 turnably supported in a shield frame 1, a ring gear 4 provided on the turning ring 2, a pinion 5 meshed with the ring gear 4, and a drive motor 7 supported on the shield frame 1 side to drive the rotation of the pinion 5. A sensor gear 9 is provided on the rotary shaft 8 of the drive motor 7, and first and second sensors 13 and 14 facing the sensor gear 9 are supported on the shield frame 1 side. The first and second sensors 13 and 14 are arranged at the displacement of 1/4 wavelength while a set of a recess and a projection 10 and 11 of the sensor gear 9 are arranged at the displacement of 1 wavelength, and an arithmetic section 15 calculating the turning direction and the turn angle of the turning ring 2 with the outputs of the first and second sensors 13 and 14 is provided.