In one embodiment, a multi charged-particle beam writing apparatus includes a plurality of blankers switching between ON and OFF state of a corresponding beam among multiple beams, a main deflector deflecting beams having been subjected to blanking deflection to a writing position of the beams in accordance with movement of a stage, a detector scanning a mark on the stage with each of the beams having been deflected by the main deflector and detecting a beam position from a change in intensity of reflected charged particles and a position of the stage, and a beam shape calculator switching an ON beam, scanning the mark with the ON beam, and calculating a shape of the multiple beams from a beam position. A shape of a deflection field of the main deflector is corrected by using a polynomial representing an amount of beam position shift that is dependent on a beam deflection position of the main deflector and then the mark is scanned with the ON beam. The polynomial is different for each ON beam.
