Belt-pulley systems are widespread in the industry due to their high efficiency and their low cost. However, there are only few works about the monitoring of their degradation. This paper deals with belt looseness detection. It presents an innovative method based on the application of a square-wave speed reference in order to monitor belt looseness. A statistical-based indicator is computed from the Fourier series coefficients of the pattern provoked by the square wave. This indicator is tested on a 30 kW induction machine and a direct-current machine coupled with two trapezoidal belts for two speeds and two load conditions. Results are compared to those obtained with a more classical harmonic approach. It reaches very good results and allows 100% detection of strong belt looseness faults without any false alarms. Moreover results are robust to speed and load variations.