The Izu-Bonin island arc is a typical oceanic island arc formed by subduction of the Pacific plate beneath the Philippine Sea plate. Although some crustal structures of this island arc have been identified, detailed information about the region between the deeper crust and uppermost mantle, especially the crust-mantle transition layer, has been inadequate to elucidate a crustal growth model. Comparison of the observed and synthetic waveforms of wide-angle seismic data yielded velocity contrast values of the top and bottom of this transition layer beneath the volcanic front along the northern Izu-Bonin island arc. Using those data, we clarify the nature of the crust-mantle transition layer. The top of the crust-mantle transition layer has a velocity contrast value of about 0.4 km/s in the southern area, extending from Kurose hall to Tori-shima along the northern Izu-Bonin island arc, except for the northern area, extending from Nii-jima to Kurose hall (0.25 km/s). The velocity contrast value at the bottom of this transition layer is large (>0.4 km/s) between Nii-jima and Kurose hall and small (0.2 km/s) from Kurose hall southward. These results and the average P wave velocity in the crust-mantle transition layer indicate that this transition layer beneath the volcanic front along the northern Izu-Bonin island arc is a mixture of mafic residues and olivine cumulates formed during crustal growth. Furthermore, the variation of the velocity contrast at the top and bottom of this transition layer and the average P wave velocity in this layer along this arc imply that this layer in the southern area, extending from Kurose hall southward, has a larger ratio of mantle materials (olivine cumulates) to mafic residues than that in the northern area, extending from Nii-jima to Kurose hall. The Moho along the volcanic front in the oceanic island arc probably has a complex signature, as suggested by the crust-mantle transition layer comprising a mixture of mafic residues and olivine cumulates.