PROBLEM TO BE SOLVED: To reduce the quantity of the memory while maintaining necessary calculation accuracy and also to perform fast probability integration by inhibiting production of an endpoint node when the attainment probability of a node is smaller than a prescribed constant. SOLUTION: Probability integration in the process of probability which uses a n-ary tree to be reconnected is performed. A block 510 stores a constant C in a storage device of a system. A block 520 produces a node and calculates the attainment probability of the node with the system. When the node is an endpoint node and also the attainment probability is smaller than the constant C, a block 530 inhibits node production. When the inhibited endpoint node is an upper end node, the block 530 adds the attainment probability of the inhibited endpoint node to the attainment probability of a node that is lower by one than the inhibited endpoint node. When the inhibited endpoint node is a lower end node, it adds the attainment probability of the inhibited endpoint node to a node that is upper by one than the inhibited endpoint node.
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