Cloud-to-ground lightning flashes in a coastal area of the Sea of Japan have been observed for a few years by a magnetic direction-finding system for lightning return strokes. Through the operation evaluation of the system, the data obtained so far have been carefully analyzed and corrected. In winter the activity of lightning does not move inland farther than 20--30 km from the coastline, and most of the flashes are detected on the sea. Seasonal variations are also found in polarities and magnetic signal strengths of lightning return strokes. ¿ American Geophysical Union 1989