A multi charged particle beam writing method includes dividing a maximum irradiation time per a shot into a digit number of first irradiation time periods, each of which is calculated by multiplying a corresponding second gray scale value by the quantization unit, where second gray scale values are gray scale values defined in decimal numbers converted from each digit value of data of binary numbers; dividing second irradiation time periods, which are a part of the first irradiation time periods into third irradiation time periods; dividing irradiation of each beam into the first irradiation steps of the third irradiation time periods and second irradiation steps of the remaining undivided first irradiation time periods; and irradiating a target object, in order, with the multi beams such that the groups are respectively composed of combination of at least two irradiation steps of first irradiation steps and second irradiation steps and the groups continue in order.