CO2 efflux in the period of snow cover can be a large carbon source in the yearly carbon budget of snowy ecosystems. However, the behavior of CO2 in snowpacks and the mechanisms of the snow surface efflux are still unclear. We performed continuous (half-hourly) midwinter measurements of CO2 concentrations in a conifer-broadleaf mixed forest snowpack, and found that concentrations in the snowpack fluctuated significantly as wind speeds varied. The snow surface efflux was evaluated as the sum of the CO2 storage change in the snowpack and the CO2 input from the soil to the snowpack, taking into account the mixing due to airflow. The median value over 52 days (49 mmol m-2 d-1) was almost the same as the daily net ecosystem exchange rate in this forest (50 mmol m-2 d-1) estimated by the eddy covariance technique and the storage-change flux in the air column. These values are clearly larger than the value we estimated using Fick's law of diffusion. These results show that airflow can be a dominant cause of mixing within snowpacks in midwinter. In addition, in the soil pores under the snowpack, the CO2 concentration was primarily related to air temperature, implying that soil respiration responds directly to air temperature, not to soil temperature, even beneath a 1-m-thick snowpack. We infer that the air temperature affected the root activity of trees through their trunks and that the variation in root respiration strongly affected the CO2 concentration fluctuation in soil under the snowpack.