A charged particle beam writing apparatus includes an enlarged pattern forming circuitry to form an enlarged pattern by enlarging a figure pattern to be written, depending on a shift number which is defined by the number of writing positions shifted in the x or y direction in plural writing positions where multiple writing is performed while shifting the position, a reduced pattern forming circuitry to form a reduced pattern by reducing the figure pattern, depending on the shift number, and an irradiation coefficient calculation circuitry to calculate an irradiation coefficient for modulating a dose of a charged particle beam irradiating each of small regions, using the enlarged and reduced patterns.