A shape detecting apparatus comprises a slit projecting means for projecting a slit image on a three-dimensional object such as a soldered area, a positioning means for positioning the three-dimensional object relative to the slit projecting means, an image pickup means for two-dimensionally scanning the slit image projected by the slit projecting means to pickup the image, a light segment extracting circuit including a center position extracting means for extracting a mean position (Z.sub.1 +Z.sub.2)/2 of two position signals Z.sub.1 and Z.sub.2 at which a video signal derived by transversely scanning the slit image by the image pickup means corresponds, to a first higher reference V.sub.1 when the video signal exceeds the first higher reference V.sub.1, a maximum value position extracting circuit for extracting a position Z corresponding to a maximum value of the video signal when the maximum value of the video signal is no higher than the first higher reference V.sub.1 and exceeds a second lower reference V.sub.2 and an erasing means for erasing the position signal when the maximum value of the video signal is no higher than the second lower reference V.sub.2, and a detecting means for detecting undersoldered condition and oversoldered condition by analyzing and evaluating the three-dimensional object based on the light segment position information derived from the light segment extracting circuit.