We have detected anomalous very-low-frequency earthquakes within the accretionary prism along the Nankai Trough, southwestern Japan. Centroid moment tensor inversion analysis reveals that the earthquake hypocenters are distributed at ~10 km depth above the upper surface of the subducting Philippine Sea Plate, and within 50--70 km landward of the trough axis. The focal mechanisms indicate reverse faulting. Their hypocenters are distributed beneath a deformation zone of an accretionary prism in sea-floor topography. These observations suggest that the occurrence of very-low-frequency earthquakes is related to numerous reverse fault systems within the accretionary prism, and that the earthquakes reflect the dynamics of deformation within this accretionary prism.