An oil passageway structure for an engine, which comprises: a body-side oil passage formed inside an engine body having plural cylinders; a cover-side oil passage formed inside an endless transmission band cover which is attached to a lateral surface of one end in a cylinder row direction of the engine body, and covers an endless transmission band; an oil passage coupling section (23A, 23B, 23C) in which a first mating face formed around a peripheral edge of an endless transmission band cover-facing open end of the body-side oil passage is coupled to a second mating face formed around a peripheral edge of a body-side oil passage-facing open end of the cover-side oil passage; an approximately annular-shaped groove formed in at least one of the first mating face and the second mating face; a sealing member formed in a shape corresponding to that of the groove and attached to the groove, characterized in that a cutout is formed in the at least one mating face to extend from an engine upper region-facing side of the groove outwardly in a radial direction of an associated one of the oil passages and open to an outer edge of the at least one mating face; and the sealing member is formed with an outward protrusion having a shape corresponding to that of the cutout and protruding upwardly from an outer periphery of the sealing member, beyond an outer edge-side end of the cutout, and the cutout is formed inclinedly with respect to a cylinder axis as viewed in the cylinder row direction, and in that the sealing member is formed in an annular shape, wherein the sealing member has an inward protrusion protruding radially inwardly from an inner periphery of the sealing member, the inward protrusion being provided offset with respect to the outward protrusion, in a circumferential direction of the sealing member. a body-side oil passage formed inside an engine body having plural cylinders; a cover-side oil passage formed inside an endless transmission band cover which is attached to a lateral surface of one end in a cylinder row direction of the engine body, and covers an endless transmission band; an oil passage coupling section (23A, 23B, 23C) in which a first mating face formed around a peripheral edge of an endless transmission band cover-facing open end of the body-side oil passage is coupled to a second mating face formed around a peripheral edge of a body-side oil passage-facing open end of the cover-side oil passage; an approximately annular-shaped groove formed in at least one of the first mating face and the second mating face; a sealing member formed in a shape corresponding to that of the groove and attached to the groove, characterized in that a cutout is formed in the at least one mating face to extend from an engine upper region-facing side of the groove outwardly in a radial direction of an associated one of the oil passages and open to an outer edge of the at least one mating face; and the sealing member is formed with an outward protrusion having a shape corresponding to that of the cutout and protruding upwardly from an outer periphery of the sealing member, beyond an outer edge-side end of the cutout, and the cutout is formed inclinedly with respect to a cylinder axis as viewed in the cylinder row direction, and in that the sealing member is formed in an annular shape, wherein the sealing member has an inward protrusion protruding radially inwardly from an inner periphery of the sealing member, the inward protrusion being provided offset with respect to the outward protrusion, in a circumferential direction of the sealing member.