1. A pattern generation method of a design pattern for use in circuit pattern formation, the method comprising: acquiring, by using a computer, a first design constraint for first patterns to be formed on a film by a first process, the first design constraint using, as indices, a pattern width of an arbitrary one of the first patterns, and a space between the arbitrary one of the first patterns and a pattern adjacent to the arbitrary one of the first patterns; modifying, by using the computer, the first design constraint according to a pattern conversion to obtain a second design constraint for second patterns, the pattern conversion being achieved by performing a second process to form the second patterns on the film based on the arbitrary one of the first patterns, the second design constraint using, as indices, two pattern spaces on both sides of a predetermined one of the second patterns, or widths of two patterns on both sides of a predetermined pattern space of the second patterns; judging, by using the computer, whether the design pattern satisfies the second design constraint; and changing, by using the computer, the design pattern in the case where the design pattern does not satisfy the second design constraint, so that the changed design pattern satisfies the second design constraint, wherein the second process comprises: forming second pattern formation materials on sidewalls of the first patterns; and removing the first patterns to leave the second pattern formation materials to form sidewall patterns as the second patterns.