To provide an internal combustion engine capable of suppressing the tension fluctuations of an auxiliary machine drive belt due to rotational fluctuations of a crankshaft.

An auxiliary pulley 15 is made to abut against that portion of a belt, which is closer to a drive pulley 7, for example. In the case of a two-cylinder internal combustion engine, the auxiliary pulley 15 has an elliptical shape and is synchronously rotated through a timing belt 22 and gears 18 and 19. As a result, the shorter circle portion of the auxiliary pulley 15 is brought into abutment against the belt 13, when the belt 13 is tensed, whereas the longer circle portion is brought into abutment when the belt 13 is slackened. Since the auxiliary pulley 15 is interlocked with the crankshaft 6, the auxiliary pulley 15 changes the abutment against the belt 13 properly without any timing deviation. The drive pulley 7 is completely round so that no struggle of the belt 13 occurs.