1. An apparatus for recognizing a pattern image having multiple picture elements, comprising: a first memory for storing a pattern image; pattern recognition means connected to said first memory for recognizing patterns in said pattern image according to a tree-based pattern recognition procedure including plural recognition steps; said pattern recognition means including a decision tree having a root node and a plurality of branch and leaf nodes arranged in a root-to-leaf node configuration, wherein the root and branch nodes correspond to selected picture elements of the pattern image and the leaf nodes correspond to pattern codes, and further wherein a pattern code is determined to correspond to an input pattern image by following said decision tree in a root-to-leaf node direction in accordance with the color of each picture element associated with each node encountered in the decision tree; a second memory connected to said pattern recognition means for storing a pattern image erroneously recognized by said pattern recognition procedure; means for generating a first probability map including plural storage positions, each of which stores a value representing a probability of black appearing in a picture element of said erroneously recognized pattern image; means for supplying said values as input data to said pattern recognition procedure and detecting one or more leaf nodes of said decision tree having a predetermined minimum probability of being assigned to the input pattern image during said pattern recognition procedure; means for generating a second probability map including plural storage positions, each of which stores a value representing a probability of black appearing in a picture element of a pattern image corresponding to a pattern code assigned to one of said detected leaf nodes; and means for extending said decision tree from at lease one of said detected leaf nodes by comparing said first and second probability maps, identifying a picture element wherein the probability values of each probability map have a maximum difference, assigning that picture element to said detected leaf node, and extending said detected leaf node to a new pair of leaf nodes representing, respectively, a pattern code correctly corresponding to said erroneously recognized pattern image and the pattern code previously assigned to said detected leaf node.