The northwestern part of the Hida Mountains (Northern Japanese Alps) along the Itoigawa-Shizuoka Tectonic Line is characterized by fragmentary distribution of peridotite (or serpentinite), intruded by younger granites. No documents on petrologic characteristics of the contact metamorphosed peridotite have been provided because of poor accessibility and exposition of the peridotite complex. We examined them via cobbles of the rivers cutting the contact between the Happo-O’ne peridotite complex and the Ariake granite, and found orthopyroxenes with extremely low CaO (< 0.1 wt%), which shows radial aggregates and no exsolution lamellae of clinopyroxene. This feature is exactly the same as that of metamorphic orthopyroxene in typical high-grade deserpentinized peridotites. The progressive contact metamorphism by the Ariake granite produced the olivine-orthopyroxene assemblage in the Happo-O’ne peridotite complex. Similar orthopyroxene was also found in one of peridotite boulders from the Nishina Mountains near the Aoki granite to the south of Happo-O’ne. Minerals in the Nishina peridotite sample are chemically heterogeneous, suggesting a relatively short duration of heating in accordance with the smaller size of the Aoki granite body. The usefulness of river cobbles was shown in investigation of rocks which are not easily accessible in mountainous area.