A novel technique is provided, which can secure sufficient adhesion of an Si layer and a magneto-optical material layer while avoiding occurrence of cracks when fabricating an optical nonreciprocal element by bonding the Si layer on which a rib waveguide is formed, and the magneto-optical material layer. The method includes forming a waveguide on the Si layer of an SOI substrate which is a first substrate, forming a first thin-film buffer layer on the aforesaid waveguide, forming a second thin-film buffer layer on an magneto-optical material layer deposited on a second substrate by using a same material as that of the aforesaid first thin-film buffer layer, and bonding the aforesaid first thin-film buffer layer and the aforesaid second thin-film buffer layer in placement where a light propagating in the aforesaid waveguide can be caused to generate nonreciprocal phase change by the aforesaid magneto-optical material layer.