A reactive oxygen species measuring device comprising: a reactive oxygen species sensor provided with an electrode assembly having a working electrode and a counter electrode formed into a shape enabling the detection of the presence of an in-vivo active oxygen species as an electric current, a power source means for applying a measuring voltage to said reactive oxygen species sensor, and a reactive oxygen species concentration measuring means (3, 3a) for measuring a concentration of the reactive oxygen species from said current detected by said reactive oxygen species sensor; and wherein the working electrode of said reactive oxygen species sensor includes, on its surface, an element capable of detecting the presence of active oxygen species as an electric current; characterized in that said element capable of detecting the presence of reactive oxygen species as an electric current consists of a polymerized film of a metal porphyrin complex; said metal porphyrin complex is represented by the following formula: wherein M indicates a metal ion selected from the group consisting of iron, manganese, a cobalt, chromium and iridium; at least one of four Rs is any one of thiofuryl, pyrrolyl, furyl, mercaptophenyl, aminophenyl and hydroxyphenyl groups; and each of other Rs indicates any of the above-described groups, or alkyl group, aryl group or hydrogen; and at least one of two Ls is an nitrogen-based axial ligand such as imidazole and its derivative, pyridine and its derivative, aniline and its derivative, histidine and its derivative, and trimethylamine and its derivative, a sulfur-based axial ligand such as thiophenol and its derivative, cysteine or its derivative, and methionine and its derivative, or an oxygen-based axial ligand such as benzoic acid and its derivative, acetic acid and its derivative, phenol and its derivative, an aliphatic alcohol and its derivative or water; and the other L indicates any of the above-described axial ligands or no ligand; and said reactive oxygen species concentration measuring means (3, 3a) includes an external output means for delivering the measured concentration of the reactive oxygen species to the outside of the measuring means, and the device further includes a remote monitoring means (12, 12a) separated from said external output means and adapted to monitor the concentration of the active oxygen species under the reception of the output from said external output means. a reactive oxygen species sensor provided with an electrode assembly having a working electrode and a counter electrode formed into a shape enabling the detection of the presence of an in-vivo active oxygen species as an electric current, a power source means for applying a measuring voltage to said reactive oxygen species sensor, and a reactive oxygen species concentration measuring means (3, 3a) for measuring a concentration of the reactive oxygen species from said current detected by said reactive oxygen species sensor; and wherein the working electrode of said reactive oxygen species sensor includes, on its surface, an element capable of detecting the presence of active oxygen species as an electric current; characterized in that said element capable of detecting the presence of reactive oxygen species as an electric current consists of a polymerized film of a metal porphyrin complex; said metal porphyrin complex is represented by the following formula: wherein M indicates a metal ion selected from the group consisting of iron, manganese, a cobalt, chromium and iridium; at least one of four Rs is any one of thiofuryl, pyrrolyl, furyl, mercaptophenyl, aminophenyl and hydroxyphenyl groups; and each of other Rs indicates any of the above-described groups, or alkyl group, aryl group or hydrogen; and at least one of two Ls is an nitrogen-based axial ligand such as imidazole and its derivative, pyridine and its derivative, aniline and its derivative, histidine and its derivative, and trimethylamine and its derivative, a sulfur-based axial ligand such as thiophenol and its derivative, cysteine or its derivative, and methionine and its derivative, or an oxygen-based axial ligand such as benzoic acid and its derivative, acetic acid and its derivative, phenol and its derivative, an aliphatic alcohol and its derivative or water; and the other L indicates any of the above-described axial ligands or no ligand; and said reactive oxygen species concentration measuring means (3, 3a) includes an external output means for delivering the measured concentration of the reactive oxygen species to the outside of the measuring means, and the device further includes a remote monitoring means (12, 12a) separated from said external output means and adapted to monitor the concentration of the active oxygen species under the reception of the output from said external output means.