PURPOSE: To dry a steel plate with necessary quantity of heat of min. limit and to economize energy consumption by controlling the proportion of recirculating air in accordance with various kinds of outer conditions in the titled apparatus of recirculation type.

CONSTITUTION: Sucking air 31 is sucked with a blower 1 via a damper 11 and heated with a heat exchanger 2 and sent into a gas jet chamber 3 incorporated in a dryer main body 5. Hot air is blown on the steel plates 20 conveyed continuously from the nozzles 4 of the chamber 3 to evaporated the moisture on the surface of the steel plates and the exhaust gas 32 high in humidity is discharged via a camper 12. A damper 13 is provided on the way of a recirculating air pipe 33 communicating the upstream of the damer 12 and the downstream of the damper 11 to recirculate one part of the exhaust gas. Therein, the ideal coefficient of recirculation is calculated by measuring each quantity and flow rate of the sucked air, the air of a dryer outlet and the blown-out air, the quantity and concn. of vapor contained in each air of these and each temp. of the steel plates at a dryer inlet and the blown-out air. The dampers 11W13 are regulated so that the difference between the calculated coefficient and the real coefficient of recirculation becomes within the preset allowable value.