On a radially inner surface of a sealing lip of an oil seal, radiation ray is irradiated to induce chemical reaction in the materials of the sealing lip, to form hard helical portions. The hard helical portions act like a helical groove or rib to pump oil back to oil-side to show an excellent sealing property when there is a relative rotational movement between the sealing lip and a shaft to be sealed, as well as even static sealing ability at the time there is no relative rotative movement.