Electronic WBGT measuring instruments have been widely used, and JIS standards were established in 2017. On the other hand, measurement errors based on differences in measurement principles have been pointed out, and countermeasures against such errors are urgently needed. In this study, the causes of the measurement error were analyzed, and an effective and simple correction method was investigated. The main reason for the measurement error was that the effect of solar radiation was not evaluated in the estimation of natural wet bulb temperature. A comparison of the correction equations for natural wet bulb temperature showed that the equation based on the thermal equilibrium equation of natural wet bulb was effective. Although an effective and simple correction method is necessary for the calculation inside the electronic WBGT measuring instrument, it was found that a simple and relatively effective correction can be made by using the difference between the globe temperature (Tg) and the air temperature (Ta). In the future, it will be possible to realize an electronic WBGT measuring instrument with higher reliability by using the simple correction method proposed in this study.