1. A rotation angle measuring method of a multi-charged particle beam image comprising: two-dimensionally scanning a mark arranged on a stage in a multi-charged particle beam writing apparatus using, among multi-charged particle beams that can be used for exposure, a plurality of representative beams of which number is smaller than the number of beams constituting the multi-charged particle beams; creating a two-dimensional image of the plurality of representative beams based on signals obtained by two-dimensional scanning; and acquiring a rotation angle of the multi-charged particle beam image using the two-dimensional image.