A writing pattern data generating method for, in order to form a first photomask for use in a manufacturing method of a semiconductor device which including forming a first resist pattern on a mask film formed on a first film using the first photomask, forming a first mask pattern by etching the mask film, removing the first resist pattern, forming a second resist film on the mask film, forming a second resist pattern on the mask film, forming a second mask pattern by etching the mask film, removing the second resist pattern, and forming a first film pattern by etching the first film, the generating method comprising correcting a first pattern data in accordance with a difference between the first film pattern and the second mask pattern and a difference between the first resist pattern and the first mask pattern.