PURPOSE: To enhance the sensitivity and selectivity of a tape or detecting oxidizing gas such as chlorine by the use of color reaction.

CONSTITUTION: A reacting solution in which p-n-butoxyaniline, an ultraviolet absorbent and polyatomic alcohol are dissolved in an organic solvent is prepared, and is impregnated with a paper sheet. After the organic solvent is evaporated, and accordingly, the paper sheet over which p-n-butoxyaniline and the ultraviolet absorbent are developed by means of the moisture absorbed in the polyatomic alcohol. When the paper sheet is exposed to oxidizing gas such as chlorine, the p-n-butoxyaniline is subjected to color-reaction through the moisture in the polyatomic alcohol so as to exhibit a reacting trance proportional to a density of the oxidizing gas. Meanwhile, during storage, the reaction between the p-n-butoxyaniline and light is inhibited by the ultraviolet absorbent, and accordingly, the sensitivity of the tape can be prevented from being degraded due to discoloration.