A method to quantitatively detect an optimum endpoint of dressing of a polishing pad with a non-destructive monitoring of a surface of the polishing pad is offered. The polishing pad is dressed for a predetermined period, and roughness of the surface of the polishing pad is measured with an optical measurement device made of a laser focus displacement meter. Then a characteristic curve representing a correlation between surface roughness of the polishing pad and dressing time is obtained. A gradient of the surface roughness versus dressing time characteristic curve is obtained. Dressing is stopped when the gradient reaches a predetermined value of gradient. These steps are repeated until the gradient of the surface roughness versus dressing time characteristic curve reaches the predetermined value of gradient.