PURPOSE: To enable the colorimetric determination of an analyte by reducing a nitroso compd. to an imino compd. accompanied by the enzymatic oxidation of the analyte and forming a color insensible to obstruction and capable of being easily evaluated visibly by one process by a non-oxidizable chromogen detection reagent.

CONSTITUTION: An electron-rich aromatic nitroso compd. represented by formula I (wherein R1 is H, hydroxy, substd. or non-substd. alkyl or the like; and R2 is hydroxy, alkoxy, amino or the like) is reduced to an imino compd. accompanied by the oxidation of an analyte in the presence of PQQ dependent dehydrogenase. This imino compd. forms leuco dye molecules colored by a color forming coupling reagent such as a phenol compd. or the like or converts a chromogen nitrosoaniline compd. represented by formula II (wherein R1 is H, hydroxy, substd. or non-substd. alkyl or amino; as advantageous X-Y, N=CR6, R6 is H or substd. or non-substd. alkyl) to a colored quinoidimide compd. to relate the concn. of an analyte visually or photometrically.