An exosome secretion inhibitor comprising a substance that suppresses the expression of NAPG, HINT3 or GXYLT1 gene or a substance that suppresses the activity of NAPG, HINT3 or GXYLT1 protein. The exosome secretion inhibitor enables the suppression of exosome secretion and, as a result, enables the suppression of the proliferation and metastasis of cancer cells. Thus, the substance capable of suppressing the expression of the aforesaid gene or the activity of the aforesaid protein can be provided as an active ingredient of a medicinal composition, which is appropriately usable for cancer therapy or cancer metastasis suppression, and also as an anticancer drug having a novel function mechanism of specifically suppressing the expression of the aforesaid gene or the activity of the aforesaid protein.
