This paper presents a numerical analysis and a quantitative investigation of the experimental error in temperature measurement caused by insertion of a water-cooled suction pyrometer for measuring temperature and gas concentration in a combustor. Two gas combustors of different size and input heat with and without a suction pyrometer were numerically analyzed. The major factor causing experimental error in temperature measurement is heat loss in the suction pyrometer. In a small gas combustor with low input heat of 3.71 kW, combustion phenomena in the combustor were changed by insertion of the pyrometer, and large temperature differences of about 300 K were observed. On the other hand, in a large gas combustor with high input heat of 14.8 kW, no large temperature differences were found. Thus, a water-cooled suction pyrometer should not be used for the temperature measurements in a small experimental combustor. The quantities of heat loss and heat transfer phenomena around the suction pyrometer were also investigated. By comparing convective and radiative heat transfer, the latter was found to account for about 85% of the heat loss in water-cooled suction pyrometer. In order to decrease the heat loss and the experimental error in the suction pyrometer, the heat loss in the suction pyrometer should be much smaller than the input heat. Material with large emissivity should be used for the surface of the suction pyrometer, or the thickness of the suction pyrometer should be large enough to increase its surface temperature.