An autonomous traveling unit is advanced toward a wall such that a charging unit is located on the right side. When a distance to the wall reaches a predetermined value not smaller than a thickness of a first member of the charging unit, the autonomous traveling unit is stopped. In this state, the autonomous traveling unit is rotated by 90.degree. to the right so as to face a plane formed by a second member of the charging unit at the right angle. Then, the autonomous traveling unit is advanced toward the second member of the charging unit, and when a distance to the plane reaches a predetermined value, the autonomous traveling unit is stopped. In this state, the autonomous traveling unit is rotated by 90.degree. to the right such that charging terminals face power supply terminals provided on the first member of the charging unit at the right angle. Finally, the autonomous traveling unit is run backward slightly, and when the charging terminals come into contact with the power supply terminals, the autonomous traveling unit is stopped and charging to the autonomous traveling unit is started.