The summer season in Korea is very hot and humid, and cross ventilation has been used to cool the indoors. However, when it is insufficient to cool the indoors with only cross ventilation, it is hard to sleep in at night and outside is felt relatively cooler than inside. Apartment houses in Korea used to be built using reinforced concrete. It is regarded that the reinforced concrete, which is a weight material, raises the indoor temperature higher than the outside air temperature during the night. In this study, the indoor thermal environment in an apartment house was measured to understand thermal specificities under the conditions of cross ventilation, and to find the potentiality to improve indoor thermal environment during tropical nights. The results showed that indoor air temperature follows the change in outdoor air temperature. The difference between outdoor air temperature and indoor air temperature is less than 0.7 °C because air change rate is largely dependent on the condition of cross ventilation. There may be a possibility of improving indoor thermal environment in summer by utilizing the finishing material whose thermal conductivity is lower than the presently used material.