To provide a gas sensing system in a closed waste disposal site, that can monitor a state of an environmental atmosphere in the waste disposal site by using the smallest possible number of gas sampling means and can be constructed advantageously in terms of cost.

A gas sensing system includes: a local gas sampling means for sampling gas in a local place within a closed waste disposal site; a gas sensing means; and a managing means. The local gas sampling means includes: a flexible gas sampling tube suspended from a ceiling portion of the closed waste disposal site; and a gas sampling head connected to a tip of the gas sampling tube and placed and used on a waste landfill face. A sampling point by the local gas sampling means can be freely changed and set when a worker carries the gas sampling head.