The retention of gases from soil fumigants chloropicrin (CP) and 1,3-dichloropropene (1,3-D) in andosols covered with a gas barrier film (GBF) was evaluated. Soil surfaces were covered with either a GBF or polyethylene film (PEF) after the application of a mixture of CP and 1,3-D. The gaseous concentrations of CP and 1,3-D were periodically measured at the soil surface (under the films) and in the soil at 10 and 20 cm depths up to 10 days after the application. The gaseous concentrations of both fumigants under the GBF were higher than those under the PEF at all measurement points. Significant differences were recognized by 6 days after application at the latest. The values of concentration×time (CT), defined as the time-integrated concentration of CP and 1,3-D, for soil under the GBF were 48–214% greater than those for soil under the PEF. These results indicate that covering fumigant-treated andosol with a GBF results in higher retention of CP and 1,3-D and achieves higher CT values than covering fumigant-treated andosol with a PEF; therefore, greater efficacy of soil fumigants can be expected.