To provide a method for simultaneously measuring rotational speed of each pulley built on a belt drive and using obtained measured data to decompose a slip ratio of a belt into a plus-side component of the slip ratio and a minus-side component thereof on the basis of speed of the belt.

This method comprises an input step S1 for inputting rotational pulse waveforms in the form of analog signals A1 (t1 to tn) to An (t1 to tn), an A-D conversion step S2 for converting the analog signals into digital signals D1 (t1 to tn) to Dn (t1 to tn), an ASCII code conversion step S3 for converting the digital signals into ASCII codes C1 (t1 to tn) to Cn (t1 to tn), an array computing step S4 for reading the ASCII codes, a step S5 for calculating rotational speeds by computing the array, and a step 6 for calculating slip ratios based on the rotational speeds.