Since 2020, sufficient ventilation in rooms has been required for prevention of COVIT-19 transmission. If space cooling is done together with sufficient ventilation, electric power use of air conditioner increases and cooling efficiency goes down, because space cooling is almost entirely done by air conditioners in Japanese houses and increase of ventilation rate makes cooling load large. In radiant cooling, radiators absorb not only convective heat from room air, but also radiant heat from indoor radiant environment such as walls, floor, and so on. Therefore, we can expect that the decline in cooling effect due to the use of radiant cooling combined with natural ventilation is not as large as that of air conditioning combined with natural ventilation. However, it is very rare for houses to have radiant cooling system and houses realizing both radiant cooling and natural ventilation are even more unusual. From such a point of view, the purpose of this paper is to grasp the actual condition of air and thermal environments in the residential building realizing both radiant cooling and natural ventilation. Followings are results obtained from this study. 1) As the resident’s actual scheme for operating both radiant cooling and natural ventilation, he adjusts radiator temperature and air volume rate of small fans behind radiators so that room temperatures are kept 28 oC at most. He is also using an external shading (bamboo screen) on the south window and continuously opens windows about 10cm. 2) Combination between radiant cooling and natural ventilation enables the resident to make CO2 concentrations in rooms 1000 ppm or less and room air temperatures and MRTs 28 oC or less. 3) The high temperature outside air is moving near the ceiling due to its small density. Therefore, the maximum temperature difference between 1.6m above the floor and the floor surface is 0.4 oC and it does not cause thermal discomfort, so that the cooling effect below the position 1.6m above the floor cannot be disturbed by warm outside air. 4) Estimated ventilation-rate varies from 26 to 385 m3/(h･person) and those values are larger than recommendation ventilation-rate for prevention of COVID-19, 30 m3/(h･person) (SHASE, The Society of Heating, Air-Conditioning and Sanitary Engineers of Japan), except 17 hours during the field measurement period.