Abstract: A high-level synthesis apparatus synthesizes a large-scale logic circuit. The apparatus has a unit for generating a control description graph according to a behavioral description graph written in a behavioral description language; a unit for sorting the control description graph according to control conditions and extracting single flows including partial graphs or closed loops from the sorted control description graph; a unit for providing an initial circuit; a unit for dividing the single flows into execution steps; a unit for allocating hardware parts of the initial circuit to the execution steps; and a unit for converting each of the single flows into a finite state machine and combining the finite state machines into one. This apparatus optimizes each single flow and adds parts to or modifies the initial circuit, to synthesize a large-scale circuit.