PURPOSE: To form a portable low cost exposure indication paper and to accurately detect the gas intensity without wetting the indication paper by a constitution wherein the elongated indication paper changing color by a specific kind of gas is covered with a transparent film, while an exposure section is formed on an edge portion thereof.

CONSTITUTION: An elongated potassium iodide starch paper 2 (indication paper) that changes its color by exposure to a specific kind of gas, e.g. ozone, is covered with transparent films 1, 3 to form an exposure section 2a to be exposed to outer air on an edge portion thereof. A part 3a of the film 3 on the exposure section of the indication paper is removed. When exposing it over a prescribed period of time in the ozone gas, the gas is diffuses into the inside from the exposure section 2a so that the indication paper 2 (potassium iodide starch paper) changes color. The length of a transition interval corresponds to an amount of exposing of the ozone gas, thereby it is possible to easily detect the intensity of ozone by reading the length of the transition interval with a scale 4.