In surgical devices, it is pointed out that tissues around these devices may experience thermal damage (coagulation). Therefore, evaluating the coagulated region generated by a microwave surgical device is an important issue. In this paper, we estimate the distribution of the coagulated region using numerical calculation. Then, there is a possibility that the calculated results will be affected by change to the electrical and thermal constants during heating. Therefore, the temperature dependence of the tissue was considered. As a result, it was confirmed that the coagulation width at the center of the lower blade was 0.5 mm larger than that without temperature dependence.