1. An apparatus for visualizing a Boolean expression, the apparatus comprising: an input section inputting a Boolean expression to be visualized in a form of a binary tree in which a leaf node represents an operand in the Boolean expression and a node other than the leaf node represents an operator in the Boolean expression; a transforming section transforming the binary tree into a two-dimensional nested representation composed of a plurality of regions; a drawing section drawing a pictorial representation for visualization on the basis of the nested representation; a storage section storing the binary tree input by the input section, the nested representation created by the transforming section, and the pictorial representation drawn by the drawing section; and a display section visually displaying the pictorial representation.