To provide a method and an apparatus in which the exposure amount of a gas is detected with good operability.

A specific kind of gas which flows in a conduit 10 is diffused toward one end from the other end of an indicator 13. An indicator layer 16 on the indicator 13 is discolored in a part which comes into contact with the gas, i.e., toward one end from the other end of the indicator 13. When the indicator 13 is irradiated with light from a light source 20, a quantity of transmitted light which is transmitted through a discolored part in the indicator layer 16 and that which is transmitted through an undiscolored part are different while a discoloration boundary is boundary is used as a boundary. Consequently, a light receiving element 21 is moved by a movement means 22 along the indicator 13 together with the light source 20, the transmitted light is received by the light receiving element 21, and the discoloration boundary can be detected. Since the position of the discoloration boundary is proportional to the exposure amount of the gas, the exposure amount of the gas can be detected on the basis of the discoloration boundary.