PURPOSE: To make possible the holding of the balance of an outlet steam temperature of 4-systems of superheaters automatically.

CONSTITUTION: Metal temperatures of superheaters A1, A2, B1 and B2 are detected separately. Deviation signals 13A of the A1 and A2 are inputted into a PI controller 15A through a dead band 14A to obtain A1/A2 bias signals 8. The bias signals 8 are added to an opening command signal 5b to generate an opening command signal 16b while being subtracted from an opening command signal 5a to generate an opening command signal 16a. Thus, superheater spray control valves 1 and 2 controlled so that the opening thereof increases on the high temperature side to lover the metal temperature and it decreases on the low temperature side to raise the metal temperature.