Given a Bayesian network relative to a set I of discrete random variables, we are interested in computing the probability distribution PA or the conditional probability distribution PA|B, where A and B are two disjoint subsets of I. The general idea of the algorithm of successive restrictions is to manage the succession of summations on all random variables out of the target A in order to keep on it a structure less constraining than the Bayesian network, but which allows saving in memory ; that is the structure of Bayesian Network of Level Two.