To provide an oil passage structure in which a third member is applied on a first member on a side that faces a second member and an internal oil passage is formed by passages inside such members and in which the sealing property of the parts where the second member abuts the first member and the third member is secured without expanding the outer diameter of the third member while securing the easiness of installing the third member.

The internal oil passage is formed by a passage 42 inside a cylinder head 1, a passage 43 inside a front cover 2, and a passage of a check valve unit 50 mounted from the side of a diameter expanding concave section 44, an annular groove 4 that extends across the abutting part with the cylinder head 1 and the abutting part with the check valve unit 50 is installed on the abutting side of the front cover 2, and an elastic sealing member 80 provided with an outer lip 81 that is attached firmly to the cylinder head 1 and an inner lip 82 that is attached firmly to the check valve unit 55 is mounted to the annular groove 4.