If an insulated and airtight house is cooled by passive ventilation using buoyant convection, the indoor air temperature can be kept lower than the outdoor air temperature, but there is a tendency for the indoor humidity to remain at a high level. In this study, the thermal environment in a house in which a dehumidifier is used in summer is numerically calculated, and the performances of various dehumidifiers are examined. The results shows that a moisture-absorbent type dehumidifier, which has a low cooling load, can create a comfortable indoor thermal environment in summer. Moreover, in the case of insulated and airtight houses, the use of a moisture-absorbent type dehumidifier enables the indoor environment of the house to be controlled by the use of natural energy instead of a heat pump.