To achieve simple and easy evaluation by equipping tetrazolium salt coloring matter or the like whose color changes due to contact with active oxygen.

An active oxygen specimen for detecting active oxygen species contained in body fluid or the like is created by soaking a solution obtained by dissolving, for example, nitro-blue-tetrasolium salt indicated by the expression III that is a tetrazolium slat synthetic coloring matter indicated by the expression I (in the expression, R1 and R2 indicate hydrogen, halogen, or a monohydric organic group, and R3 indicates a divalent organic group) or an expression II (in the expression, R1, R2, and R3 indicate hydrogen, halogen, or a monohydric organic group) into specific buffer liquid into filter paper or the like for drying with wind. Then, when a sample is dripped, formazan is generated by active oxygen species for changing into a color corresponding to the concentration. As the tetrazolium slat coloring matter, 2-benzothiazolyl-3-(4-carboxy-2- methoxyphynyl)-5[4-(2-sulfoethylcarbamoyl)phenyl-2H-tetrazolium, and the like are also used.