PURPOSE: To efficiently use control rod drive mechanisms by selecting many of them to drive them by a few power supply units and providing spare power supply units.

CONSTITUTION: For example, a controller 9, a power supply unit 10, a spare power supply unit 11, a control rod switcher 12 and a man-machine device 5 are provided for 205 control rod drive mechanisms 1. By selecting drive electricity outputted by the power supply unit 10 on signals from the controller 9 in response to information commands from the man-machine device 5 for each of a few of the control rod drive mechanisms 1 operated simultaneously and supplying it to them with the control rod switcher 12, the control rods are moved to target positions to control the operation of a nuclear reactor. During the failure of the power supply unit 10 or the full insertion of all control rods, moreover, a net working rate is improved by actuating the spare power supply unit 11 automatically according to the increase of a load in case of the simultaneously operation of the control rod drive mechanisms 1.