An internal combustion engine is provided for a personal watercraft in which a portion of the engine that is enlarged to accommodate a balance shaft is minimized in size, whereby the internal combustion engine is made compact. Balance shafts for preventing secondary vibration are arranged together with a crankshaft on a dividing plane between a crankcase and a cylinder block of an internal combustion engine. The balance shafts lie mutually in parallel with the crankshaft. A crankshaft bearing is formed in the dividing plane on each of plural mutually opposed, longitudinally spaced rib pairs, and balance shaft bearing is formed in the dividing plane on a selected rib pair of the plural rib pairs. The balance shaft bearing is positioned so as to be biased in a direction in which the balance shaft bearing is closer to an adjacent rib compared with the crankshaft bearing.