A beam writing apparatus according to an embodiment includes a selection unit configured to select a dose equation from a plurality of dose equations for calculating a dose of a beam, for each small region of a plurality of small regions made by virtually dividing a writing region of a target workpiece into mesh-like regions, a dose calculation unit configured to calculate a dose of a beam which is shot into a small region of the plurality of small regions, by using a selected dose equation, for the each small region, and a writing unit configured to write a desired pattern in the small region, by using a calculated dose, for the each small region.