A main bearing for a crankshaft of an internal combustion engine is composed of a pair of semi-cylindrical bearings. An oil groove extending in a circumferential direction is formed on an inner circumferential surface of one semi-cylindrical bearing. The oil groove has the depth gradually decreasing toward a circumferential end surface of the one semi-cylindrical bearing, and defines a circumferential end portion on the circumferential surface without extending to an axial groove formed in a joint portion of the pair of semi-cylindrical bearings. An extension portion of the inner circumferential surface extends between the circumferential end portion and the axial groove, and a plurality of circumferential grooves extending in the circumferential direction is formed on the extension portion from the oil groove to the axial groove over an entire width of the circumferential end portion of the oil groove.