An exosome secretion inhibitor containing a substance suppressing expression of NAPG, HINT3 or GXYLT1 gene, or a substance suppressing activity of NAPG, HINT3 or GXYLT1 protein. It is possible to suppress secretion of exosome, thereby making it possible to suppress proliferation or metastasis of cancer cells, so that the substance suppressing expression of the gene or activity of the protein can be used as an active ingredient of a pharmaceutical composition suitably used in treatment of cancer or suppression of cancer metastasis, and also the substance can provide a carcinostatic agent having a new action mechanism which specifically suppresses expression of the gene or activity of the protein.
