Method and apparatus for detecting vertical and horizontal deformations of leads of a semiconductor device by illuminating the leads with a planar light beam and determining relative positions of the planar light beam on the leads on the basis of distribution of light rays reflected from the leads. Different lead deformations can be detected with one and the same mechanism within a short time. By applying the invention to a semiconductor device mounting apparatus, a single detector can be employed in common both for the detection of the semiconductor device position and the detection of reflections of the planar beam.