Outbreaks of gall inducers sometimes cause serious damage to their host plants. In recent decades, the gall midge Schizomyia castanopsisae Elsayed & Tokuda, 2018 (Diptera: Cecidomyiidae) inducing inflorescence galls on Castanopsis sieboldii (Makino) Hatus. (Fagaceae) occurred at extremely high densities in southern Izu Islands, Japan, and led to severe loss of host acorn production. This gall midge is suspected to have recently invaded the Izu Islands from the Nansei Islands, and the absence of associated parasitoids may cause the outbreaks there. Although this gall midge was not found in the northern Izu islands nor in the Izu Peninsula in early 2010, northward range and population expansions have been suspected in later surveys. In this study, we investigated the galls of S. castanopsisae in northern Izu islands and compared the changes in population density in this decade. As a result, the density of galls was noticeably increased on Ohshima, Niijima, and Kozushima Islands. Parasitoids were not found in galls collected from these islands. We consider that the outbreaks of S. castanopsisae are gradually expanding northward in the Izu Islands.