A method of surface roughness measurement is disclosed which uses a fiber-optic probe having a sensor head constituted of a light-emitting fiber and multiple light-receiving fibers disposed coaxially with the light-emitting fiber. The surface roughness measurement includes the steps of: (a) pre-measuring a first correlation between detected intensity and gap distance for each of reference samples obtained under a plurality of different processing conditions; (b) searching a second correlation between maximum intensity and surface roughness for each of the reference samples, based on the first correlation, and storing the second correlation in memory; (c) adjusting the gap distance so as to set the probe in a position at which the maximum intensity is obtained for the reference sample machined under the processing condition to be monitored; and (d) monitoring the maximum intensity of the machined surface at the gap distance set in step (c), and determining the roughness of the machined surface based on the second correlation.