PURPOSE: To lighten low cycle fatigue of turbine casings, by supplying cooling water to the casings wherein the cooling water was heat-exchanged with the exhaust of the low pressure turbine of a two-stage pressure up type turbocharger.

CONSTITUTION: A heat exchanger 19 is provided for the exhaust side of a low pressure turbine 15. Cooling water which was heat-exchanged with the exhaust of the turbine by the heat exchanger 19 is supplied to the casings of the low pressure turbine 15 and a high pressure turbine 14 via flow control valves 21 and 23 so as to cool the casings. The temperature difference between the turbine casings and the cooling water is made smaller so as to lighten thermal stress. Thus, low cycle fatigue occurring by thermal stress can be lightened.