A piston-crank mechanism links crankpins of a crankshaft with piston pins of pistons by using a plurality of links. The piston-crank mechanism allows an upward inertia force produced near a top dead center of each piston to be smaller than a downward inertia force produced near a bottom dead center of the piston in order to reduce secondary vibration occurring during operation. In a four-cycle inline four-cylinder internal combustion engine, a total force of inertia forces exerted from adjacent cylinders to each of second and fourth crankshaft bearings becomes a downward force, which reinforces a downward force produced in response to combustion pressure. These second and fourth crankshaft bearings have a rigidity higher than the remaining crankshaft bearings.