A surface roughness measuring unit according to the present invention includes a contact pin unit having a contact pin and a displacement detector, the contact pin being provided so as to project and retract through a through-hole of a skid and scanning and moving along a surface of a work piece, the displacement detector detecting displacement of the contact pin in a direction perpendicular to the work piece surface; a driver moving the contact pin unit forward and backward along the surface of the work piece; and a joint coupling the contact pin unit and the driver to a measurement head holder of a coordinate measuring system. The surface roughness measuring unit further includes a contact detector that detects contact of the skid on the surface of the work piece.