A main bearing for rotatably supporting a journal section of a crankshaft of an internal combustion engine, the journal section comprising a cylindrical body portion, a lubricating oil path extending to penetrate the cylindrical body portion, and at least one inlet opening of the lubricating oil path that is formed on an outer peripheral surface of the cylindrical body portion, wherein the main bearing comprises first and second half bearings (41, 42) which are adapted to be combined in a cylindrical shape by butting circumferential end surfaces of the respective half bearings (41, 42), the first and second half bearings (41, 42) are configured so as to form together an axial groove extending through an entire length of the main bearing in an axial direction on an inner circumferential side of each butting portion when the first and second half bearings (41, 42) are combined, each half bearing (41, 42) comprises a main cylinder portion including a circumferential central portion of the half bearing (41, 42), and crush relief portions which are formed over the entire length in the axial direction at both circumferential end portions of the half bearing (41, 42) so that a wall thickness of the crush relief portion is thinner than that of the main cylinder portion, and each crush relief portion extending from the circumferential end surface toward the circumferential central portion of the half bearing (41, 42) with a center angle not less than 3° but not more than 15°, a plurality of crush relief grooves (74, 75) extending in the circumferential direction over an entire length of the crush relief portion are formed in each crush relief portion so as to communicate with the axial groove, an oil groove is formed only on an inner circumferential surface of the first one of the first and second half bearings to extend in the circumferential direction at least over the main cylinder portion of the first half bearing, a circumferential end portion of the oil groove on a front side of a rotation direction of the crankshaft opens on the circumferential end surface of the first half bearing on the front side of the rotation direction, a circumferential end portion of the oil groove on a rear side of the rotation direction of the crankshaft is located in the crush relief portion of the first half bearing on the rear side of the rotation direction, characterized in that the first half bearing further comprises a transition region formed between each of both crush relief portions and the main cylinder portion so that a wall thickness of the transition region becomes thinner toward the crush relief portion from the main cylinder portion, the second half bearing further comprises a transition region formed at least between the crush relief portion on the rear side in the rotation direction of the crankshaft and the main cylinder portion so that a wall thickness of the transition region becomes thinner toward the crush relief portion from the main cylinder portion, and each of the transition regions comprises an inward protruding curved surface in a protruding shape which protrudes inward in a radial direction, seen from the axial direction of the half bearing (41, 42).