UnconjugatedN-glycans (UNGs) of the high mannosyl-type extracted from tomato fruit delay ripening of fruit pericarp discs by 2 days when applied exogenously. Since the amount of high mannosyl-type UNGs decreases while the amount of complex-type UNGs increases during early ripening, we postulated that high mannosyl UNGs are converted to complex-type by Golgi enzymes to relieve inhibition of ripening-inhibition by the high mannosyl-type UNGs. In this paper, we investigated the effect of deoxymannojirimycin, an inhibitor of mannosidase I involved in complexN-glycan synthesis. Deoxymannojirimycin, applied to pericarp discs, both inhibited synthesis of complex-typeN-glycans and delayed ripening, supporting our hypothesis on modulation of tomato ripening by UNGs.