Radon and thoron exhalation rates from the ground were measured at 23 sites in Tokyo and nine prefectures of Japan that were selected in consideration of geological features and soil distribution. The in situ measurements were carried out with a ZnS (Ag) scintillation detector equipped with a photomultiplier and a container measuring 30×40cm and 11cm in depth. The arithmetic means of radon and thoron exhalation rates were estimated to be 7.6±2.6mBq·m^-2·s^-1 and 0.69±0.16Bq·m^-2·s^-1, respectively. The exhalation rates differed up to 37-fold for radon and 265-fold for thoron between the maximum and minimum values, respectively. A geological interpretation suggests that the regional variations of the exhalation rates were mainly controlled by the source rock of the soil distributed around the measuring site. The correlation coefficients between the exhalation rate and SiO₂ content of the rocks and soils distributed around the sites were estimated to be 0.99 for radon and 0.90 for thoron, respectively. Geological information is therefore considered to be a good indicator for estimating the exhalation rate. In addition, a correlation coefficient of 0.78 was accepted between the radon and thoron exhalation rates. This is considered to be useful information for making a rough estimation of one exhalation from the other.