PURPOSE: To improve the accuracy and susceptabililty of verification by effectively preventing the oxidation of indicating dye existing in indicating reagent composition before the contact of the composition with the test sample by using predetermined composition.

CONSTITUTION: Peroxide active substance like hemoglobin or myoglobin in urea or the other test sample is determined or qualitatively verified by using oxidation preventing indicating reagent composition containing phosphorus compound which contains indicating dye, metallic ion complex, hydroperoxide, 2 or more free acid functional value. The phosphorus compound has general structure of X=P(OR') (OH)2, or X=P(R2), (OH)2, where R1 or R2 is hydrogen, the residue of 1-12C unsubstituted or substituted ariphatic part, unsubstituted or substituted aromatic part and ariphatic or aromatic polyvalent compound, and X is O, S or NH. The early oxidation of the dye can be excluded by using the reagent composition.