1. An assertion generating system that generates an assertion description which is used for assertion verification of a semiconductor integrated circuit, comprising: a specification inputting unit that generates design data or specifications and a document for confirming a specification of the semiconductor integrated circuit by graphically editing the specification of the semiconductor integrated circuit based on user operations; a computer readable storage device having a first storing unit that stores the design data generated by the specification inputting unit; a property generating unit that generates a property which verifies the specification of the semiconductor integrated circuit by reading the design data generated by the specification inputting unit from the first storing unit and using the read design data; a second storing unit that stores the property generated by the property generating unit, wherein the second storing unit may or may not be a part of the computer readable storage device; and an assertion generating unit that automatically converts the property into an assertion description if the property is to be verified during assertion verification by reading the property generated by the property generating unit from the second storing unit, wherein the property generated by the property generating unit is a selection condition with respect to a state transition, a logic value of at least one or more signals, or at least one or more signals in the design data.