A discrimination machine optically senses an object having a surface with a planar structure while scanning the planar structure along the surface. The machine includes a discrimination sensor including optical devices detecting light generated from the planar structure. The optical devices are disposed at an interval in a transverse direction, perpendicular to a scanning direction, to ensure a sufficiently wide sensing area for the object. A deviation detector detects deviation of the planar structure from a plane based on electrical signals output from the respective optical devices detecting the light generated by the object while the discrimination sensor is scanning the object. An optical device selector selects an optical device, from among the optical devices, based on the deviation detected. A determination is made as to whether the electrical signal output from the optical device selected is within an allowable margin.