This paper examines whether the trade policy can be justified when positive externality exists in agricultural sector. Import tariff as a trade policy is compared with production subsidy throughout this paper to prove its validity by using graphical treatment and mathematical analysis. The result demonstrates that trade policy as well as production subsidy can be justified in that it could improve social welfare in the presence of positive externality, contradicting the previous work's argument by Paarleberg et al. that trade policy interventions are never justified by multifunctionality. Although import tariff is inferior to production subsidy its validity itself can never be denied judging from the criterion of Pareto improvement. The results also show that the significance of import tariff, however, tends to decrease with the demand more elastic, the supply less elastic, and the import dependency becoming larger.