A gas sensor includes a catalyst layer and a pipe-shaped thermoelectric power generation device. The pipe-shaped thermoelectric power generation device includes an internal through-hole along the axial direction of the pipe-shaped thermoelectric power generation device, a plurality of first cup-shaped components each made of metal, a plurality of second cup-shaped components each made of thermoelectric material, and first and second electrodes disposed at the ends of the pipe-shaped power generation device. The plurality of the first cup-shaped components and the plurality of the plurality of second cup-shaped components are arranged alternately and repeatedly along the axial direction. The catalyst layer is arranged on the internal surface of the internal through-hole. A method for detecting or measuring gas by using the gas sensor includes supplying a fluid containing the gas into the internal through-hole of the gas sensor, and detecting voltage between the first and second electrodes.