PURPOSE: To provide a tension adjusting method for a transmitting belt whereby its tension can be adjusted to a proper value relating to a load of a driven device without stopping a machine during its operation, in the case of the machine in which power of a driving gear of electric motor or the like is transmitted to the driven device of serving as a work device through the transmitting belt of a V belt or the like.

CONSTITUTION: A proper tension value of a V belt 2 relating to a new load of a work device 3 is input to a servo controller 36 to drive a servo motor 32 by an output of the servo controller 36. A distance between an electric motor 1 and the work device 3 is varied by action of the servo motor 32 to change tension of the V belt 2. Here, the tension of the V belt 2 is monitored by a load sensor 26 and the servo controller 36, to which an output of the sensor 26 is input, and fed back to the servo controller 36.