1. A synthesis constraint creating device that creates synthesis constraint information indicating a constraint which must be satisfied when a behavioral synthesis device creates a register transfer level description from a behavior level description, the synthesis constraint creating device comprising: a process emergence number acquiring unit that classifies a process described in the behavior level description in accordance with a process attribute, and acquires, for each process attribute, an emergence number of a process belonging to each process attribute, the process being in the behavior level description; a circuit structural component specifying unit that specifies, for each process attribute, a kind of a circuit structural component which can embody a process belonging to each process attribute and which can be described in the register transfer level description based on library information for specifying circuit structural components which can be described in the register transfer level description; and a synthesis constraint information creating unit that creates synthesis constraint information indicating a kind and a number of a circuit structural component necessary for expressing a process described in the behavior level description by the register transfer level description based on the emergence number of a process acquired by the process emergence number acquiring unit for each process attribute, and the kind of a circuit structural component specified by the circuit structural component specifying unit for each process attribute.