A crankshaft of an in-line, four-cylinder internal combustion engine has five journal portions. The journal portions are each supported by a main bearing unit. Each of the main bearing units includes a cylinder block side bearing part and a bearing cap. The No. 1, No. 2, No. 4 and No. 5 main bearing units employ an iron-based bearing cap, and the No. 3 main bearing unit at the center employs a bearing cap made of an aluminum alloy having a relatively high thermal expansion coefficient. At high temperatures, the bearing clearance of the No. 3 main bearing unit is configured to expand so as to reduce bearing load.