PURPOSE: To obtain a required surface roughness in accordance with a nature of an object or the like by summing crest components and trough components which are obtained from differences between respective averaged values of heights of crests and depths of troughs up to a predetermined number and the height of the center line position.

CONSTITUTION: A surface roughness measuring unit 11 delivers surface roughness data and a center line position of a measured magnetic recording medium, as two- or three-dimensional position coordinates, to a surface-roughness computing device 10 which selects heights of crests (depths of troughs) up to n/2-th thereof (where n is set by the user in accordance with the kind of an object of the like), counting from the highest (lowest) one. The heights of the crests up to n/2-th are averaged, and the averaged value is subtracted from the height of the center line position so as to obtain a trough component Rnv. Further, components Rnp, Rnv are summed up so as to obtain an n-point averaged surface roughness R,.. Thus obtained Rrz, Rnv, Rnz are suitably stored in an auxiliary memory device, and are outputted to a printer 12 as necessary.