A multi charged particle beam writing apparatus includes a dividing circuitry to divide first irradiation time data into k pieces of second irradiation time data, where each of the k pieces of second irradiation time data has a different number of bits and the total of different numbers of bits is n-bits, in multi-pass writing of k or more passes by using multi charged particle beams, a data transmission circuitry to transmit, for each of k passes, corresponding second irradiation time data, for a beam concerned in multi charged particle beams, a resolution information transmission circuitry to transmit, for each of k passes, corresponding resolution information, and an irradiation time calculation circuitry to calculate an irradiation time of a corresponding beam in the multi charged particle beams of a pass concerned by using input second irradiation time data and resolution information.