More than ten en echelon cracks, ranging from 100–700 meters in length, successively formed on alluvial deposits and continued to creep during higher seismicity periods of the Matsushiro swarm earthquakes in 1966. Some of these cracks are still creeping at present with a velocity of a few millimeters per year. The en echelon cracks, especially those with left‐lateral displacement, define a straight zone, 0.1–0.5 km wide and 4 km long, and are again arranged en echelon. The amount of displacement along the zone is estimated by various field measurements at about 50 cm in the left‐lateral component and 10–20‐cm uplift of the southwest side. The double en echelon cracks in the zone are best interpreted as surface traces of a left‐lateral strike‐slip fault newly formed in the bed rock. There are other evidences of the faulting on the northwest extension of the zone. They are the left‐lateral shift of railroad tracks in an interval of a few hundred meters and systematic breakage of a network of dikes between the rice paddy.