When biomass fuel reacts with oxygen for a long period of time, an amount of acid ions in a tank is increased with time. However, when metallic ions are generated for some reason and the reaction proceeds to change the acid ions into metallic salts, the increase rate of the amount of the acid ions in the fuel tank becomes slow. When the metallic salt forming reaction proceeds rapidly, the amount of the acid ions may even be reduced. Therefore, in the embodiment, the change of the acid ions into the metallic salts in a fuel is detected. Unless the fuel is newly supplied by refueling, the decrease of the amount of acid ions in the fuel can be considered as a change into metallic salts. Accordingly, the generation of metallic salts can be detected by monitoring the decrease of the amount of acid ions in the fuel. Thus, clogging of a fuel supply system or the like caused by the metallic salt forming reaction can be prevented.