A multi-cylinder internal combustion engine comprising: first and second crankshafts disposed apart from and parallel with each other; first and second pistons operatively connected to the first and second crankshafts through connecting rods, respectively; and first and second cylinders for slidably accommodating therein the first and second pistons, respectively, both the crankshafts being interlocked with each other to rotate synchronously, the first and second cylinders being disposed adjacent to each other in the axial directions of the crankshafts. Vibrations due to the primary inertial force can be eliminated or alleviated without using a special balancer shaft. The size and weight of the engine are greatly reduced as compared with a conventional horizontally opposed type or V type engine.