The purpose of this study is to figure out the characteristics of thermal environment in a workshop at school in Japan and to propose the improvement method of the thermal environment without air-conditioning systems. In this paper, measurement results of thermal environment in the workshop and calculation results of vertical temperature gradient are shown. In the measurement results, indoor air temperature became very high in summer. Solar radiation was the main factor raising the temperature of PC roof, and large vertical temperature gradient was formed. The calculation results of zonal model shows the good agreement with the measurement results for vertical temperature gradient. Then some improvement methods of the thermal environment was investigated with the calculation model, and the investigations showed that insulating PC roof is effective method to decrease indoor air temperature.