Osmium isotope is a sensitive tracer of crust and sediment as its eroding form, because of significant contrast between high Os isotope ratio of .crust and sediments and low ratio of mantle. Therefore, Os isotope provides information on contribution of slab component to the island arc lavas and assimilation of continental crust to the source magma during magma ascent. However, whether the frequently observed high Os isotope ratio of arc basalt is derived from slab component or from the overlying continental crust is highly debated. Here we report significantly high 187Os/188Os of Cr-spinel of Mukoo-jima, Izu-Bonin arc. As Cr-spinel is resistant to later alteration and weathering and is the early stage crystal in the fractional crystallization, it preserves the information of very primitive magma without any contamination of late stage crustal assimilation during magma ascent. Cr-spinel of Yome-shima and Chichi-jima island boninites yield low 187Os/188Os of 0.1232 and 0.1242, the lowest values among those of island arc rocks, whereas that of Mukoo-jima Island has extremely high 187Os/188Os of 0.1429. It strongly suggests contribution of the slab component into source magma of Mukoo-jima island and no possible slab component to the source magma of Chichi-jima and Yome-shima boninites.