PURPOSE: To provide the oil seal structure of a pressure sensor improving the durability of an O-ring and dispensing with a backup ring.

CONSTITUTION: A pressure sensor 1 has a pressure sensitive module 15 for measuring pressure by the deformation of a diaphragm 6. An O-ring 5 is disposed at the pressure receiving face 13 of the pressure sensitive module 15 and the peripheral part of the opposite face. Since a boundary face 18 which is a narrow clearance between the periphery of the pressure sensitive module 15 and a housing 2 is interposed between the pressure receiving face 13 and the O-ring 5, pressure change in the O-ring 5 is slow and low. The durability of the O-ring 5 is thereby improved, and a backup ring for preventing the collapse of the O-ring 5 is dispensed with.