A metal compound (50) able to trap a sulfur component in exhaust gas is arranged in a flow path of the exhaust gas, a property of the metal compound (50) changing along with an increase of the amount of sulfur component trapped at the metal compound (50) is measured, and the cumulative value of the amount of SOX actually contained in the exhaust gas is calculated from the measured property. On the other hand, the assumed cumulative value of the amount of SOx, assumed to be contained in the exhaust gas when assuming that fuel or oil of a sulfur concentration assumed in advance is used, is calculated. It is judged if fuel or oil with a high sulfur concentration is being used from the actual cumulative value of the amount of SOX and the assumed cumulative value of the amount of SOx.