A multi charged particle beam writing apparatus of the present invention includes an aperture member to form multiple beams, a plurality of first deflectors to respectively perform blanking deflection of a corresponding beam, a second deflector to collectively deflect the multiple beams having passed through the plurality of openings of the aperture member so that the multiple beams do not reach the target object, a blanking aperture member to block each beam that has been deflected to be in the off state by the plurality of first deflectors, and a current detector, arranged at the blanking aperture member, to detect a current value of all beams in the on state in the multiple beams that have been deflected by the second deflector.