1. A pattern design method of designing a pattern of a semiconductor circuit to be formed on a substrate, comprising: creating a design pattern by placing a plurality of cells in each functional block as a unit of the semiconductor circuit and executing routing among the plurality of placed cells; creating mask pattern data based on the design pattern; predicting a predictive pattern to be formed on the substrate by the mask pattern data; checking a difference amount between the predictive pattern and a target pattern to be formed on the substrate by the mask pattern data; comparing the difference amount with a predetermined allowable variation amount; and correcting at least one of placement and routing of the cells in the design pattern corresponding to the mask pattern data used to predict the predictive pattern if the difference amount is larger than the allowable variation amount in the comparing.