PURPOSE: To achieve a stable measurement by a method wherein a pH-sensitive element fixing a matrix in which a metal complex composed of a compound having a specific structure and of iron has been dispersed is used as a high- sensitivity sensor with reference to a superoxide.

CONSTITUTION: An ion (bivalent) complex in which tetrakis(2-pyridimethyl)- diamino compound represented by the formula (where R2 is hydrogen or a methyl group) has been coordinated is fixed to a pH-sensitive element. The iron complex of the diamino compound has the catalytic action of a chemical reaction. For example, since hydrogen ions are consumed in a reaction, a pH change is caused. That is to say, the consumed amount of the hydrogen ions is proportional to the amount of a superoxide which exists, the magnitude of the pH change depends on the existing amount of the superoxide. As a result, when the iron complex of the diamino compound represented by the formula is combined with the pH-sensitive element, the superoxide can be measured.