A processing method in which microchips, each provided with at least an electrophoretic flow path (54, 55) performing migration of a sample for analysis inside a sheet-like member, comprising: pretreatment steps being performed using a dispenser common to the multiple electrophoretic flow paths (54, 55), each of the pretreatment steps comprising at least one of addition of a size marker or a fluorescent reagent, filling of a separation buffer solution, injection of a sample into the electrophoretic flow path (54, 55), and cleaning the electrophoretic flow path; electrophoresis steps, each of which subsequent to the pretreatment step; and wherein the pretreatment steps are performed such that when a pretreatment step for one electrophoretic flow path (54, 55) ends, a pretreatment for a next electrophoretic flow path (54, 55) starts, and wherein each of the electrophoresis steps is performed for a corresponding electrophoretic flow path (54, 55) independently of the other electrophoretic flow paths (54, 55) using a respective electrophoresis high voltage power supply part (26-1,···26-4) capable of applying a migration voltage to each of the electrophoretic flow paths (54, 55) independently of the others, and the electrophoresis steps are sequentially performed partly simultaneously to one another with different start times among the electrophoretic flow paths (54, 55), wherein each of the electrophoresis steps includes a separation step of electrophoresing the sample introduced at one end of the separation flow path in the electrophoretic flow path (54, 55) in a direction toward the other end thereof due to a voltage applied between both ends of the electrophoretic flow path (54, 55) to thereby separate the sample into components thereof; and a detection step detecting the separated components of the sample. pretreatment steps being performed using a dispenser common to the multiple electrophoretic flow paths (54, 55), each of the pretreatment steps comprising at least one of addition of a size marker or a fluorescent reagent, filling of a separation buffer solution, injection of a sample into the electrophoretic flow path (54, 55), and cleaning the electrophoretic flow path; electrophoresis steps, each of which subsequent to the pretreatment step; and wherein the pretreatment steps are performed such that when a pretreatment step for one electrophoretic flow path (54, 55) ends, a pretreatment for a next electrophoretic flow path (54, 55) starts, and wherein each of the electrophoresis steps is performed for a corresponding electrophoretic flow path (54, 55) independently of the other electrophoretic flow paths (54, 55) using a respective electrophoresis high voltage power supply part (26-1,···26-4) capable of applying a migration voltage to each of the electrophoretic flow paths (54, 55) independently of the others, and the electrophoresis steps are sequentially performed partly simultaneously to one another with different start times among the electrophoretic flow paths (54, 55), wherein each of the electrophoresis steps includes a separation step of electrophoresing the sample introduced at one end of the separation flow path in the electrophoretic flow path (54, 55) in a direction toward the other end thereof due to a voltage applied between both ends of the electrophoretic flow path (54, 55) to thereby separate the sample into components thereof; and a detection step detecting the separated components of the sample.