An installation structure for a gas sensor capable of detecting gas concentration in a highly accurate manner is provided. The installation structure for a gas sensor which detects concentration of gas circulating inside an outlet-side piping comprises a through hole 18 in an inner wall of the outlet-side piping and the gas sensor comprises a gas inlet portion with one face open within the outlet-side piping 14, and the gas sensor is installed to the outlet-side piping in a condition where the gas inlet portion does not protrude from the inner wall of outlet-side piping.