1. A learning apparatus for building, based on learning data, a network structure of a Bayesian network in which a cause and effect relationship between plural nodes is represented by a directed graph, the learning apparatus comprising: storage means in which the learning data is stored; and learning means for building the network structure based on the learning data; wherein the learning means prepares an initial population of individuals constituted by individuals each having a genotype in which orders between the nodes and cause and effect relationship have been stipulated, repeatedly performs processing for crossovers and/or mutations on the initial population of individuals based on a genetic algorithm, calculates an evaluated value of each of the individuals based on the learning data, searches for an optimum one of the individuals, and takes a phenotype of the optimum individual as the network structure.