In order to determine the causes of Sick House Syndrome (SHS), a field survey on indoor air quality and occupants' health conditions was investigated in 60 sick houses in Japan. Investigation results show that indoor air in many sick houses was polluted with high carbonyl compounds and VOC concentrations over the guidelines. Concentration is higher not only in houses just after completion or rebuilding, but also in those with high air tightness and low air change rate. Next, based on analyses with t-test, the concentrations of formaldehyde, toluene, ethylbenzene, xylene, p-dichlorobenzene and TVOC in houses of "SHS" is higher than that in houses of "non-SHS". Finally, follow-up survey shows that there were the cases where the improvement of indoor air quality has relieved occupant's symptoms.