PURPOSE: To improve hermetic seal performance, by providing a flat face contactable with the outer circumferential face of shaft to the lip of an oil seal and aligning one edge of the flat face with a plane connecting with the center of a spring.

CONSTITUTION: An inclination is formed through cutting on one inner circumferential face at the inner circumferential side of the body 12 of an oil seal 11, and a flat face 13 which functions as a lip is formed at the top portion. It is arranged such that one edge of the flat face 13 passes through a face which connects between the flat face 13 and the center of a spring arranged at the rear side of a step section 14. Consequently, a peak contact pressure is exhibited at the edge portion between a reciprocating shaft 8 and the flat face 13 contactable therewith, resulting in the improvement of hermetic seal effect.