Abstract: In a system for adjusting the tension of an endless transmitting belt in an internal combustion engine, a control system automatically adjusts the tension to be applied from a tensioner to the endless transmitting belt. The control system includes a driving-side rotational state detecting device for detecting the rotational state of a driving wheel which drives the endless transmitting belt. A driven-side rotational state detecting device detects the rotational state of a driven wheel driven by the endless transmitting belt. A control unit calculates a difference in rotational phase between the driving and driven wheels from results of detection by both rotational state detecting devices and controls the operation of a tension changing device in accordance with the difference in rotational phase. Thus, it is possible to accurately and quickly accommodate a very small variation in tension of the transmitting belt without specially using a strain gauge.