PURPOSE: To obtain a simple method by dipping an Ehrlich reagent in an indole water solution expressed by a specified formula to measure color formation of a solution, and absorbing a required quantity of reagent having enough water solubility as an object reagent in a small piece format.

CONSTITUTION: In a water solution, reagent is put together with indole expressed by the formula [R is -(CH2)nSO3-, wherein n is 2-6, or [OCH2-(CH2)x]mSO3-, wherein x is 2 or 3, and m is 1 or 2 in a water solution to confirm the existence of p-dimethylamino benzaldehyde from detectable discoloration. That is, in the form of a water solution, it is determined whether forecast discoloration is caused or not only by simply dipping a reagent piece containing p- dimethylaminobenzaldehyde and hydrochloric acid in a solution containing indole. At this time, indole is included in a carrier matrix, preferably it is used in a small piece format, so that it is possible to obtain a simple method for introducing indole into a test solution only by dipping the small piece in water.