The purpose of the present invention is to grasp the state in which hydrophilic groups of an electrolyte are distributed in a reaction layer for fuel cells. Nitric acid groups are bonded to hydrophilic groups (sulfone groups) contained in a reaction layer for fuel cells, and metal ions capable of forming a nitrosyl complex with the nitric acid groups, e.g., ruthenium ions, are introduced into the reaction layer to dye the nitric acid groups bonded to the hydrophilic groups contained in the reaction layer. When the hydrophilic groups have agglomerated, the nitric acid groups bonded thereto also agglomerate. When said nitric acid groups are dyed with ruthenium, the ruthenium also agglomerates to make it possible to examine said nitric acid groups with an electron microscope.