We attempted to improve the power generation performance of a thermoelectric device by making rectangular incisions in it to increase the temperature difference between the hot and cold ends of the device, while keeping the electrical resistance low. By means of experimental measurements and numerical simulations, dependences of the temperature difference between both ends, the generated electric power and the conversion efficiency of the device on the hot-end temperature and the incision sizes were examined. The numerical results were in good agreement with the experimental ones, and the validity of the calculations was confirmed. The numerical analyses revealed that the increase of the horizontal, rather than the vertical, size of the incision was effective in increasing the temperature difference between both ends. Furthermore, optimized incision sizes were obtained for each hot end temperature. The generated electric power was maximized to 41.4 mW by the device with the optimized incision sizes, and improved by 39.9% in comparison with that without incisions. On the other hand, the conversion efficiency was maximized to 0.82% and improved by 39.0%.