PURPOSE: To provide a sealing unit in which the sealing performance is prevented from deteriorating regardless of a tip having rectangular radial cross- section.

CONSTITUTION: The difference between the outer dimension (rt') at the corner 81 of a lip 8 and the inner dimension (rp) at the fitting face part (inner peripheral face) P of a body to be fitted (tubular part of connector) is set larger than the difference between the outer dimension at the side 81 of the lip 8 and the inner dimension at the side of fitting face part of the body to be fitted. This structure prevents the leak preventive function at the corner part 81 of the tip 8 from deteriorating. The sealing unit can be fitted easily because the height of the tip 8 is not increased over the entire periphery thereof.