In medaka, we found a C16orf35-like gene in the region within 1 Kbp 3′ downstream of the ψβ end of the 36-Kbp embryonic globin gene cluster (^5′α0^3′–^3′β1^5′–^5′α1^3′–^5′β2^3′–^5′α2^3′–^3′α3^5′–^5′β3^3′–^3′β4^5′–^5′α4^3′–^3′ψα^5′–^5′ψβ^3′). Intron 5 of the gene contained a region having NF-E2 binding sites located between GATA boxes. The region was homologous to human HS-40 in terms of the existence and structure of characteristic transcription-factor binding sites and was named Ol-HS-40. Injection of the fusion gene construct Ol-HS-40-α0(up-2)GFP, consisting of Ol-HS-40, a 5′ upstream 200-bp minimum promoter for α0, and green fluorescent protein (GFP), showed that Ol-HS-40, as in human HS-40, had the ability to strongly enhance GFP expression in erythroid cells of embryos. Further analysis using transgenic technology revealed that Ol-HS-40 had the ability to change the type of the GFP expression from embryo-to-young fish to embryo-to-adult. In addition, the results suggest that Ol-HS-40, although its natural function remains unclear, has strong enhancer activity for the expression of not only the α-globin gene but also the β-globin gene.