Paleomagnetic studies have been made on the Lower Paleogene Yorii Formation, which is distributed on the Sambagawa belt in the northeastern margin of the Kanto Mountains and is considered to be a constituent of the Atokura Nappe originated from the Inner Zone of the Southwest Japan, based on the lithology of conglomerate.Fifty-seven paleomagnetic samples were collected from 6 sites of fine-grained sandstone. Through progressive thermal demagnetization tests, 6 reliable mean paleomagnetic directions with reversed polarity before folding were obtained. Overall mean directions are D=200.8°, I=-46.0°(α₉₅=5.6°, κ=143.4). The paleomagnetic direction indicates 20.8°clockwise rotation.The restored paleomagnetic declination is different from the 94°clockwise rotation of the Kanto Mountains since 15 to 16 Ma. The difference suggests that the Yorii Formation is the Nappe from the Inner Zone of the Southwest Japan and is the allochthonous terrane having rotated counterclockwise by 73°during the nappe-forming process.