A method of optimizing a microparticle suction condition, using a microchip having a main flow channel through which a liquid containing a microparticle flows, a microparticle suction flow channel arranged coaxially with the main flow channel in a flow direction of the liquid, and a branch flow channel branching from the main flow channel, the method comprising: providing a branch point specifying process of specifying a branch point at which the branch flow channel branches from the main flow channel; characterized in that the branch point specifying process includes: a trajectory image acquisition process of acquiring a fluorescence image indicating a trajectory of the microparticle flowing through the main flow channel and the branch flow channel by continuous light emission; an image extraction process of dividing the fluorescence image into a plurality of blocks to binarize the divided blocks, and extracting a block including an image of the trajectory; and a branch point acquisition process of specifying an end block corresponding to an end portion of the main flow channel from among the blocks including images of the trajectory, and setting a barycentric position of an image of the end block as the branch point; and the method further comprising: providing a time assignment process of assigning a first time (T 1) at which suction of the microparticle is to be applied, on basis of a distance between a position of the microparticle at a time point, when a predetermined time (T X) has elapsed from a time when the microparticle passed through a predetermined position on the main flow channel, and the branch point. providing a branch point specifying process of specifying a branch point at which the branch flow channel branches from the main flow channel; characterized in that the branch point specifying process includes: a trajectory image acquisition process of acquiring a fluorescence image indicating a trajectory of the microparticle flowing through the main flow channel and the branch flow channel by continuous light emission; an image extraction process of dividing the fluorescence image into a plurality of blocks to binarize the divided blocks, and extracting a block including an image of the trajectory; and a branch point acquisition process of specifying an end block corresponding to an end portion of the main flow channel from among the blocks including images of the trajectory, and setting a barycentric position of an image of the end block as the branch point; and a trajectory image acquisition process of acquiring a fluorescence image indicating a trajectory of the microparticle flowing through the main flow channel and the branch flow channel by continuous light emission; an image extraction process of dividing the fluorescence image into a plurality of blocks to binarize the divided blocks, and extracting a block including an image of the trajectory; and a branch point acquisition process of specifying an end block corresponding to an end portion of the main flow channel from among the blocks including images of the trajectory, and setting a barycentric position of an image of the end block as the branch point; and the method further comprising: providing a time assignment process of assigning a first time (T 1) at which suction of the microparticle is to be applied, on basis of a distance between a position of the microparticle at a time point, when a predetermined time (T X) has elapsed from a time when the microparticle passed through a predetermined position on the main flow channel, and the branch point.