The coast guard is questioning three people found in a wooden boat off Okinoshima Island on Friday to establish if they are North Korean defectors, government officials said, adding that a body was also found in the vessel.
The boat had Korean characters on its hull.
One of the three was quoted as telling the coast guard that the boat drifted toward Japan after its engine quit during a fishing operation.
Last September, nine defectors from North Korea were rescued from a boat off the Noto Peninsula in Ishikawa Prefecture.
The nine were sent to South Korea a month later.