An auxiliary burner configured to heat exhaust gas on an upstream side of any one of heat exchangers, and a fuel supply system configured to supply fuel to the auxiliary burner. To avoid damage of a heat-transfer pipe by exhaust gas of high temperature, a reference value on which the restriction on the charging quantity to be supplied to the auxiliary burner is based is set, and the charging quantity of fuel to the auxiliary burner is restricted so as not to exceed a limit defined by the reference value.