To reduce the processing time of assignment operation processing by unnecessitating to repeatedly performing plural processing such as to check a variable value and to trace the address of a selection destination of a '0' or '1' branch when a logical function is expressed by a BDD(binary decision graph) and a assignment operation is performed.

A BDD representing a logical function is preliminarily stored in an executable machine language format in a BDD data storage memory 220a in a storage device 200a, a BDD processing program and BDD data that are respectively stored in a BDD processing program storage memory 210a of the device 200a and the memory 220a are directly read by a basic processor 100a, and an assignment operation that obtains a logical function value by assigning '0' or '1' to a variable of the logical function expressed by the BDD can fast be performed by decoding and executing the BDD data.