A gas analysis device includes a probe tube, a flange, an optical system member, and heaters. The probe tube includes an optical path through which measurement light is projected onto a prescribed measurement region of a sample gas flowing through a flue and/or is received from the measurement region. The flange is fixed to the outer periphery of the probe tube and is attached to a pipe side wall. The optical system member projects measurement light onto the sample gas S within the measurement region and/or receives measurement light from the measurement region. The heaters are disposed within the flange and heats the portion where the probe tube and flange are fixed to each other.