PURPOSE: To properly control an amount of refuse incinerated through estimation of the calorific value of refuse, based on an amount of a water content, by a method wherein, through detection of oxygen concentration at the outlet of an incinerator, oxygen concentration in smoke passage, and an amount of the air for combustion, an amount of water content in exhaust gas from the incinerator is measured by means of an on-line system.

CONSTITUTION: Oxygen concentration is detected by a fuel passage oxygen concentration detector 9, is transmitted by a fuel passage oxygen concentration transmitter 10, and is indicated by a fuel passage oxygen concentration indicator 11. Meanwhile, oxygen concentration at the outlet of an incinerator or in exhaust gas from the incinerator is detected by a detector 4 for oxygen concentration in exhaust gas at the outlet of the incinerator, is transmitted by a transmitter 5 for oxygen concentration in exhaust gas at the outlet of an incinerator, and is indicated by an indicator 6 for oxygen concentration in exhaust gas at the outlet of an incinerator. A value, indicated by the indicator 6 for oxygen concentration in exhaust gas at the outlet of an indicator, is converted into a dry base by a computing part 7, based on data indicated by an indicator 3 for an amount of the air for combustion and the indicator 11 for oxygen concentration in gas in a fuel passage to indicate it on an indicator 8 for water content in exhaust gas.