A crankshaft bearing support structure, comprising: a cylinder block having a first holding portion; a first plain bearing held by the first holding portion of the cylinder block; a crank cap fastened to the cylinder block, the crank cap having a second holding portion; a second plain bearing held by the second holding portion of the crank cap; and a crankshaft having an axis of rotation, the crankshaft being rotatably held by the first plain bearing and the second plain bearing and configured to receive an explosive load from a piston via a connecting rod, wherein the crankshaft flexes, when subjected to the explosive load, in a direction of the explosive load, and wherein the crank cap is provided with a mechanism which causes the second holding portion to flex in a manner that a bearing holding surface of the second holding portion follows flexure of the crankshaft subjected to the explosive load, the mechanism includes pairs of bolt-fastening points (21a1, 21a2, 22a1, 22a2) provided in the crank cap, the bolt-fastening points in the pairs being located on opposite sides of the second holding portion in an axial direction of the crankshaft and being located on opposite sides of the crankshaft when viewed from the axial direction of the crankshaft, and the bolt-fastening points in the pairs are provided in projecting portions (21, 22) projecting from opposite surfaces of the second holding portion in the axial direction of the crankshaft and integrally formed with the second holding portion, and the crank cap includes surfaces (21f, 22f) facing the cylinder block, characterized in that said surfaces (21f, 22f) are provided with grooves (26, 27) between the second holding portion and the projecting portions, the grooves extending along a longitudinal direction of the second holding portion. a cylinder block having a first holding portion; a first plain bearing held by the first holding portion of the cylinder block; a crank cap fastened to the cylinder block, the crank cap having a second holding portion; a second plain bearing held by the second holding portion of the crank cap; and a crankshaft having an axis of rotation, the crankshaft being rotatably held by the first plain bearing and the second plain bearing and configured to receive an explosive load from a piston via a connecting rod, wherein the crankshaft flexes, when subjected to the explosive load, in a direction of the explosive load, and wherein the crank cap is provided with a mechanism which causes the second holding portion to flex in a manner that a bearing holding surface of the second holding portion follows flexure of the crankshaft subjected to the explosive load, the mechanism includes pairs of bolt-fastening points (21a1, 21a2, 22a1, 22a2) provided in the crank cap, the bolt-fastening points in the pairs being located on opposite sides of the second holding portion in an axial direction of the crankshaft and being located on opposite sides of the crankshaft when viewed from the axial direction of the crankshaft, and the bolt-fastening points in the pairs are provided in projecting portions (21, 22) projecting from opposite surfaces of the second holding portion in the axial direction of the crankshaft and integrally formed with the second holding portion, and the crank cap includes surfaces (21f, 22f) facing the cylinder block, characterized in that said surfaces (21f, 22f) are provided with grooves (26, 27) between the second holding portion and the projecting portions, the grooves extending along a longitudinal direction of the second holding portion.