Completely serpentinized ultramafic rocks occur in Cretaceous sedimentary rocks in the North Shimanto belt, southwest Japan. The serpentinites are divided into bastite serpentinite and massive serpentinite. Chromian spinels in the bastite serpentinites are characterized by Cr#[= Cr/(Cr + Al)] and X_Mg[= Mg/(Mg + Fe²⁺)] corresponding to those of residual harzburgites from forearc regions. On the other hand, chromian spinels in the massive serpentinite show high Y_Fe[= Fe³⁺/(Cr + Al + Fe³⁺)] and low TiO₂ content, indicating that it was a cumulate formed by island arc magmatism. Petrological characteristics of the serpentinites from the North Shimanto belt are similar to those of the Mineoka-Kobotoke belts of the Circum-Izu area, the South Shimanto belt. The serpentinites from the North Shimanto belt were possibly fragments of the lower crust to upper mantle materials beneath an island arc, which intruded into sedimentary rocks after the accretion and emplacement of the North Shimanto belt.