In a test piece for detecting a nitrite by virtue of the diazo coupling reaction, which test piece has supported on a substrate an effective amount of at least one oxidizing agent selected from the group consisting of M'XO, M'XO 3 , M'XO 4 , M' 3 H 2 XO 6 , wherin M' denotes an atom capable of being converted into a monovalent cation and X denotes a halogen atom, ethylenediamine tetraacetic acid-metals, cuprous chloride, copper sulfate, copper acetate, mercury acetate and bismuth acetate in corporation with a diazotizing agent and a coupling agent.