High-resolution observations with a towed--conductivity-temperature-depth system were conducted along the Kuroshio Extension, where the Kuroshio and the Oyashio merge and new North Pacific Intermediate Water (NPIW) is formed. Large-amplitude frontal waves at an intermediate depth corresponding to a density range of NPIW (26.6--26.9σ$theta$) were discovered here along the main stream of the Kuroshio Extension. A remarkable salinity minimum due to the intense intrusions of Oyashio water was observed near the trough of the frontal wave with an apparent wavelength of ~100 km. Around the troughs (crests) of the frontal waves, the horizontal velocity in the intermediate layer was directed to the southern Kuroshio water side (northern Oyashio water side) relative to the upper layer. Horizontal velocity fields converged (diverged) around the wave trough (crest), indicating downwelling (upwelling). The amplitude of the frontal waves was observed to grow in the downstream direction, where isolated eddies were observed. This suggests that the development of frontal waves and wave breaking enhance isopycnal mixing and cross-frontal exchange across the main stream of the Kuroshio Extension.