Soil CO₂ concentration has been measured continuously since 1999 in two different deciduous forests, which are broad-leaved and coniferous (larch) forests developing on the same type of volcanic soil in Hokkaido, the northernmost island of Japan, and diffusive CO₂ efflux from the soil surface (soil respiration rate) was evaluated by using the Fickfs first law from the vertical gradients of CO₂ concentration and diffusion coefficients of gaseous CO₂ in the soil. Soil CO₂ concentration has been measured directly with small infrared gas analyzers (Vaisala, GMD20) buried at several depths. Gaseous CO₂ diffusion coefficients were calculated by substituting air-filled porosity of the soil for the relational expression made in a laboratory experiment by using the diffusion chamber method (Currie, 1960; Osozawa, 1987). Air-filled porosity was obtained from volumetric water content measured continuously. Moreover, CO₂ production rate of the soil surface layer of 15 cm thickness (A horizon) and CO₂ flux from C horizon to A horizon were calculated by considering the CO₂ budget and the change of CO₂ storage in A horizon; C horizon lies just under A horizon. We will report the behaviors of soil respiration rate and soil CO₂ concentration for about three years and compare them measured in two different forests.