An oil seal (1A) has an outer member (2) fitted to a crankcase (100), an inner member (3) integrally rotatably fitted to a crankshaft (101), and a first movable lip (4) disposed in the gap (G1) formed by the outer member (2) and the inner member (3). The first movable lip (4) is disposed such that it is not in contact with the outer member (2) when rotation of the crankshaft (101) is stopped and that it is displaced in the direction to approach the outer member (2), coming into contact with the outer member (2) to close the gap (G1).