To perform the quantitative analysis of iron oxyhydroxide by heating a solid iron oxyhydroxide at a specific temperature under an anaerobic atmosphere and measuring the reduction in weight.

A solid iron oxyhydroxide is heated at a temperature of 150-400°C under an anaerobic atmosphere, a reduction in weight due to own dehydration reaction is measured, and the quantitative analysis of iron oxyhydroxide is performed. When the temperature of an iron oxide to be measured increases to approximately 150°C, an adhesion water is completely eliminated, the dehydration reaction of iron oxyhydroxide starts at approximately 196°C and ends at approximately 319°C. Considering these conditions, the iron oxide is heated at a temperature of approximately 150-400°C, thus obtaining information only on iron oxyhydroxide selectively even if magnetite and hematite are mixed. With this method, the absolute value of the iron oxyhydroxide in the iron oxide can be grasped and the environment in a plant can be evaluated accurately.