1. An apparatus for inspecting a solder joint, said apparatus comprising: a solder joint image extracting means for extracting three-dimensional image information of a solder joint subjected to multi-color illumination; and a solder joint shape classifying means for learning a classification hierarchy of solder joint images by using a training set of said solder joint images classified into said classification hierarchy by a human solder joint inspector, and for applying said learned classification hierarchy to said extracted image information, said solder joint shape classifying means including a preprocessor which converts said three-dimensional image information into a plurality of one-dimensional vector sequences, each one-dimensional vector sequence being for one color of the multi-color illumination and for a one-dimensional spatial extent, calculates correlation values among all combinations of said one-dimensional vector sequences, and supplies said correlation values as said training set.