A non-transitory, recording medium stores therein a program that causes a computer to execute extracting from hardware description of a circuit, a conditional branch statement representing a conditional branch process; determining whether the extracted conditional branch statement includes at least three condition expressions, where a given combination thereof has exclusive satisfying conditions; extracting from the conditional branch statement determined at the determining, a combination of condition expressions for which satisfying conditions are exclusive; extracting each condition expression from the extracted combination and creating, for each extracted condition expression and according to an order of appearance in the hardware description, a conditional branch statement in which the extracted condition expression has a hierarchical relationship with a condition expression not included in the combination; generating an assertion for checking whether a specified condition is satisfied in each created conditional branch statement; and outputting, as assertion data of the circuit, the generated assertion.