PURPOSE: To miniaturize a plant equipment so as to enable it to correspond to diversified utilization of exhaust heat by taking out surplus heat out of a cell cooling water system in steam high in heat utilization indirectly in a manner of separating from the cell cooling water system.

CONSTITUTION: For performing exhaust heat collecting operation by generating steam from a steam generator 5, when a control valve 11 is closed to open an inlet control valve 9, low temperature water in a secondary warm water generating system is heated to saturated water by cell cooling water of flowing in a primary side of an exhaust heat collecting system preheater 4, and this saturated water flows into a low temperature side of the generator 5. Then in the generator 5, saturated water in a secondary steam generating system is heated by surplus heat of a cell cooling water system to generate steam supplied to an exhaust heat utilizing device 15, and condensed water after utilizing exhaust heat is returned to the secondary warm water generating system of the preheater 4 by a condensed water circulating pump 16. The preheater 4 is provided with good thermal conductivity because of heat exchange between water and water, and accordingly the preheater 4 can be suppressed to the small volume. Since a general heat exchanger or the like can correspond to the preheater, the cost and volume can be reduced.