A balancing mechanism for an internal-combustion engine includes a balancing shaft capable of being disposed adjacent to the crankshaft of the internal-combustion engine to enable compact construction. Balance weights are disposed with each balance weight positioned between crank webs of a crankshaft so as to correspond to a crankpin. The balancing shaft has journals having a diameter D A and a middle portion has a smaller diameter D B and is provided with a thrust flange formed near the right journal so as to be in sliding contact with the inner side surface of a right-hand bearing portion of the crankcase. A portion of the crank web corresponding to the thrust flange is recessed to form a depression. The balancing shaft driving mechanism includes a driving gear securely mounted on the crankshaft, and a driven gear securely mounted on the balancing shaft and in mesh with the driving gear. A particular tooth of the driving gear brought into mesh with a particular tooth of the driven gear are marked respectively with identical locating marks to coincide with each other when a point on the tooth surface of one of the particular teeth corresponding to the middle point between the tip and the root of the same particular tooth and a point on the tooth surface of the other particular tooth corresponding to the middle point between the tip and the root of the same particular tooth are in contact with each other.