1. A multi charged particle beam exposure method comprising: calculating an effective irradiation time, in a case where an irradiation position of each beam in multi-beams of each pass which indicates each exposure processing in multiple exposure is exposed during a control irradiation time being a desired irradiation time by continuously performing a combination of divided shots selected from a plurality of divided shots each having any one of a plurality of sub control irradiation time periods preset and being corresponding to the control irradiation time onto a same irradiation position, for each of a plurality of control irradiation time periods for controlling an irradiation time of each beam in the multi-beams of a charged particle beam, using a blanking error time of each divided shot of the plurality of divided shots, previously acquired, due to an error of blanking control for the each divided shot; generating correlation data which represents a relation between the control irradiation time and the effective irradiation time; selecting, using the correlation data, for each irradiation position of a target object, the combination of the divided shots corresponding to the effective irradiation time to be closer to the each desired irradiation time; and performing exposure, using the multi-beams, based on the combination of the divided shots selected for the each irradiation position of the target object.