According to one embodiment, a mask manufacturing device includes a positional-deviation calculating unit that acquires positional deviation information between an actual position of a pattern formed on a mask substrate and a design position decided at the time of designing the pattern to a predetermined area of a square on the mask substrate; an irradiating-condition calculating unit that calculates an irradiating condition including an irradiating amount and an irradiating position of radiation to correct the positional deviation calculated to the predetermined area of a square on the mask substrate by using positional-deviation correction information, which indicates a relationship between the irradiating amount and the irradiating position of the radiation to the mask substrate and a pattern position change after irradiation of the radiation; and an irradiating unit that irradiates the mask substrate with the radiation under the irradiating condition calculated by the irradiating-condition calculating unit.