PURPOSE: To raise a thermal efficiency of a whole gasturbine plant by using as a heating medium water having absorbed heat of exhaust gas of the gas turbine, for a heater for heating fuel oil.

CONSTITUTION: Water in a water storage tank 32 is introduced to a space in a water leading cover 34 on both sides of an exhaust gas duct 22 interconnecting to an exhaust gas outlet of a gas turbine via a water distributing pipe 36 by means of 2 set of water pump 41, and while it is rising through the space, it is heated by a heat exchange with the exhaust gas elevating its temperature. The hot water is then by means of pipe 39, sent to heaters 30, 31 where a low grade oil is supplied from pipes A, B. In this instance, the amount of water is controlled by control valves 42, 43 operated by oil temperature detectors 4, 15 attached to each of heaters 30, 31, and an amount of heat required is respectively added to heaters 30, 31. The water cooled passing through heaters 30, 31 is returned to the water storage tank 32.