A nationwide survey of indoor air quality in Japan was conducted using four types of diffusive samplers. Gaseous chemical compounds such as carbonyls, volatile organic compounds (VOC), acid gases, basic gases, and ozone were measured in indoor and outdoor air of 602 houses throughout Japan in winter and summer. Four kinds of diffusive samplers were used in this study: DSD-BPE/DNPH packed with 2,4-dinitrophenyl hydrazine and trans-1,2-bis(2-pyridyl)ethylene coated silica for ozone and carbonyls; VOC-SD packed with Carboxen 564 particles for volatile organic compounds; DSD-TEA packed with triethanolamine impregnated silica for acid gases; and DSD-NH3 packed with phosphoric acid impregnated silica for basic gases. These samplers are small and lightweight and do not require a power source, hence, it was possible to obtain a large number of air samples via mail from throughout Japan. Almost all compounds in indoor air were present at higher levels in summer than in winter. In particular, formaldehyde, toluene, and ammonia were strongly dependent on temperature, and their levels increased with temperature. The nitrogen dioxide concentration in indoor air particularly increased only during winter and was well correlated with the formic acid concentration (correlation coefficient=0.959). Ozone concentrations in indoor air were extremely low compared with the outdoor concentrations. Ozone flowing from outdoor air may be decomposed quickly by chemical compounds in indoor air; therefore, it is suggested that the indoor/outdoor ratio of ozone represents the ventilation of the indoor environment.