PURPOSE: To provide a convenient control device in a rotary air preheater in which a position of a sector plate dividing between two fluids can be always followed to a thermal deformation of a rotor and controlled.

CONSTITUTION: A lower end of a rod 5 is slidably abutted against a rotary tire 2 arranged at an outer circumference of a rotor 1 of a rotary air preheater. An upper end of the rod is connected to a spool 9 of a flow rate control type selector valve 8, and a spool is displaced in accordance with a thermal deformation of the rotor 1. Pressure fluid corresponding to a displacement direction of the spool and its amount of displacement is supplied from a pressure fluid supplying device 11 to a sector plate driving device 10 through a flow rate control direction selector valve. A piston of the sector plate driving device is connected in such a manner that the sector plate 4 is driven and displaced. In addition, the sector plate and the flow rate control direction selector valve 8 are connected. Thermal deformation of the rotor is transmitted to the flow rate control direction selector valve and the sector plate is followed in accordance with its deformation and displaced.