To prevent damage of a maintenance part and maintain high-efficient engine performance by stopping an engine in using fuel other than specified one.

This fuel monitoring device 20 of a construction machine includes an exhaust gas analyzer 21 provided in an exhaust system of the engine 10. The device includes: a measuring means 24 for measuring the concentration of a poisoning material such as sulfur dioxide contained in exhaust gas by the exhaust gas analyzer 21; a fuel discriminating means 26 for determining the use of fuel other than the specified fuel when the measured value exceeds a preset threshold; and a warning means 27 for giving a warning when another fuel other the specified fuel is used. Thus, during the use of fuel other than the specified fuel, this fact is instantaneously acquired to suitable control the operation for changing the operation of the engine 10 or the like.