A balancer mechanism for an internal combustion engine, comprising: a piston; a cylinder block (3, 303) having the piston reciprocably disposed within a cylinder chamber (3a, 3b, 3c) formed in an interior of the cylinder block (3, 303), the cylinder block (3, 303) being disposed so that an axis of the cylinder chamber (3a to 3c) is tilted in a substantially horizontal direction; a crankshaft (12, 312) accommodated in an interior space of a crankcase coupled to the cylinder block (3, 303), and rotates in synchronization with the piston; a balancer shaft that rotates in synchronization with the crankshaft (12, 312); and a balancer weight (36a, 36b) provided to the balancer shaft and rotating integrally with the balancer shaft, for canceling out vibration resulting from reciprocating motion of the piston, wherein the crankcase is formed by joining upper and lower case halves (3B, 4A, 303B, 304A) together, the upper case half (3B, 303B) is coupled to the cylinder block (3, 303), and a joining surface (12C, 312C) between the upper and lower case halves (3B, 4A, 303B, 304A) is formed so as to extend substantially vertically as seen in side view; and the balancer shaft is located below the crankshaft (12, 312), and the crankshaft (12, 312) and the balancer shaft are sandwiched and supported between the upper and lower case halves (3B, 4A, 303B, 304A) with their center axes (12d, 34d) being located on the joining surface (12C, 312C); wherein an oil pump (61, 361) and a water pump are provided to a pump shaft, the oil pump (61, 361) being driven by rotation of the pump shaft to pump a lubricating oil from an oil sump (8, 308) that stores said lubricating oil, and the water pump being driven by rotation of the pump shaft to supply cooling water to a water jacket formed in the periphery of the cylinder chamber (3a to 3c). a piston; a cylinder block (3, 303) having the piston reciprocably disposed within a cylinder chamber (3a, 3b, 3c) formed in an interior of the cylinder block (3, 303), the cylinder block (3, 303) being disposed so that an axis of the cylinder chamber (3a to 3c) is tilted in a substantially horizontal direction; a crankshaft (12, 312) accommodated in an interior space of a crankcase coupled to the cylinder block (3, 303), and rotates in synchronization with the piston; a balancer shaft that rotates in synchronization with the crankshaft (12, 312); and a balancer weight (36a, 36b) provided to the balancer shaft and rotating integrally with the balancer shaft, for canceling out vibration resulting from reciprocating motion of the piston, wherein the crankcase is formed by joining upper and lower case halves (3B, 4A, 303B, 304A) together, the upper case half (3B, 303B) is coupled to the cylinder block (3, 303), and a joining surface (12C, 312C) between the upper and lower case halves (3B, 4A, 303B, 304A) is formed so as to extend substantially vertically as seen in side view; and the balancer shaft is located below the crankshaft (12, 312), and the crankshaft (12, 312) and the balancer shaft are sandwiched and supported between the upper and lower case halves (3B, 4A, 303B, 304A) with their center axes (12d, 34d) being located on the joining surface (12C, 312C); wherein an oil pump (61, 361) and a water pump are provided to a pump shaft, the oil pump (61, 361) being driven by rotation of the pump shaft to pump a lubricating oil from an oil sump (8, 308) that stores said lubricating oil, and the water pump being driven by rotation of the pump shaft to supply cooling water to a water jacket formed in the periphery of the cylinder chamber (3a to 3c).