An internal combustion engine comprises a cylinder block including a skirt section, and a plurality of bearing sections connected to the skirt section; and a bearing beam structure secured to the bottom part of the cylinder block and including a plurality of main bearing cap sections each associating with each cylinder block bearing section to rotatably support a crankshaft, first and second beam sections disposed to connect the main bearing cap sections with each other, and first and second side wall sections disposed to securely connect the bearing cap sections with each other respectively at their one of opposite side portions and at the other one of the opposite side portions, each side wall section being securely connected at its lower part with each beam section and the its top part with the lower part of the cylinder block skirt section, thereby greatly improving the rigidity of the entire bearing beam structure, thereby effectively suppressing noise emission from the cylinder block.