To provide a technology of optimizing a suction condition of a microparticle.
The present technology provides



an optimizing method of a suction condition of a microparticle including:
a particle number counting step of detecting a time point when a microparticle passes through a predetermined position on a main flow path through which liquid containing the microparticle flows, sucking the microparticle from the main flow path to a microparticle suction flow path by the microparticle suction flow path with a predetermined suction force, and counting the number of microparticles sucked into the microparticle suction flow path; and a step of determining an elapsed time from passage through the predetermined position with which the suction by the microparticle suction flow path should be performed
on the basis of a time from the time point when the microparticle passes through the predetermined position on the main flow path until the suction is performed and the number of counted microparticles.