Heat flux sensor based on active control of temperature by a thermoelectric module is expected to be a promising instrument for convection-dominated fire tests because it can electrically adjust the temperature to its surrounding and thus enables to reduce systematic measurement error due to installation. Although the idea has already been proposed, the effect of the temperature of the thermoelectric module has not been addressed and hence a condition to properly estimate the incident heat flux is still unclear. In this study, the measurement principle is revisited and a prototype is developed accordingly. Tests with and without steady heat flux are conducted. The test without the heat flux shows that the measurement error is caused by the deviation of temperature and thus the temperature difference within the sensor from those expected by the modified Ioffe's equation. It indicates that the temperature difference needs to be zero to minimize the error. The test with the steady heat flux is then conducted to validate the proposed condition. Comparison between the measured and given heat fluxes shows good agreement within reasonable deviation, demonstrating the validity of the proposed condition.