To provide a belt life detecting system capable of predicting the life in a simple configuration even during rotation of a belt.

A toothed belt 10 is employed being wound around a pulley. The outside of a canvas 16 is coated with a conductive layer 18 to form the contact surface of the toothed belt 10 with the pulley. The canvas 16 is an insulator. Electric resistance between one part of the contact surface and another part of the contact surface is measured. Utilization of the belt causes wear of the conductive layer 18, which leads to exposure of the canvas 16 so as to increase the electric resistance to be measured. By this increase of the electric resistance, deterioration of transmission performance of the toothed belt 10 to the pulley is evaluated.