PURPOSE: To improve controllability by improving position detection accuracy in a reactor output control device controlling the control rod position of the core section of a reactor via motor drive.

CONSTITUTION: Synchronous oscillators 6a1, 6a2, 6b having different performance are installed for detecting the positions of control rods. The oscillators 6a1, 6a2 are used as a pair. Their signals are guided to comparing means 12a, 12b of controllers 10A, 10B, and the signal having better detection accuracy is normally extracted. Operation signals are generated by control arithmetic means 10a, 10b based on the target values from target generating devices 9a, 9b and the present positions of the control rods, a power control device 11 is controlled to drive a motor 4, and the control rods are extracted or inserted.