A main bearing includes first and second half bearings, each having a main cylinder portion and crush relief portions in both circumferential end portions. An oil groove circumferentially extends on an inner circumferential surface only of the first half bearing, one circumferential end portion of the oil groove opens on a circumferential end surface on a front side in a rotation direction, and the other circumferential end portion of the oil groove is positioned in the main cylinder portion. The first half bearing includes a transition region at least between the crush relief portion on the rear side in the rotation direction and the main cylinder portion, and the second half bearing includes a transition region at least between the crush relief portion on the front side in the rotation direction and the main cylinder portion.