1. An optical displacement sensor comprising: multi-imaging means including a light projecting means, an image taking means and a parameter scanning means, said light projecting means being for providing a slit beam of light and projecting said slit beam of light onto a target surface of a target object at a specified angle, said image taking means being for obtaining contour-including images including an image of a sectional contour line by an optical sectional surface by using a two-dimensional image taking element at another angle different from said specified angle to photograph said target surface where said slit beam of light is made incident, said parameter scanning means being for scanning the brightness of said contour-including images by varying the value of at least one of parameters that define image taking conditions affecting the brightness of said contour-including images obtained through said image taking means, said multi-imaging means serving to obtain a plurality of said contour-including images under different image taking conditions; image synthesizing means for extracting from a plurality of said contour-including images obtained by said multi-imaging means a segmented image satisfying a specified maximum brightness condition for each of predetermined segments and generating a synthesized image including a series of portions of the image of said sectional contour line by gathering said extracted segmented images; and measuring means for carrying out a specified measuring process based on said series of portions of the images of said sectional contour line and generating a value representing a result obtained by said specified measuring process.