1. A charged particle beam writing method comprising: inputting design pattern data; virtually dividing a writing area to be written with the design pattern data into a plurality of small areas in a mesh-like manner, and calculating a pattern density in each of the plurality of small areas based on the design pattern data; calculating a resizing amount for each pattern density in a case of irradiating a charged particle beam at an isofocal dose; resizing a dimension of the design pattern data in each of the plurality of small areas, based on the resizing amount in each of the plurality of small areas; and writing a resized design pattern on a target workpiece with the isofocal dose corresponding to the pattern density which was calculated before the resizing in each of the plurality of small areas.