Particles such as blood cells can be categorized and counted by a digital image processor. A digital microscope camera can be directed into a flowcell defining a symmetrically narrowing flowpath in which the sample stream flows in a ribbon flattened by flow and viscosity parameters between layers of sheath fluid. A contrast pattern for autofocusing is provided on the flowcell, for example at an edge of a rear illumination opening. The image processor assesses focus accuracy from pixel data contrast. A positioning motor moves the microscope and/or flowcell along the optical axis for autofocusing on the contrast pattern target. The processor then displaces microscope and flowcell by a known distance between the contrast pattern and the sample stream, thus focusing on the sample stream. Blood cell images are collected from that position until autofocus is reinitiated, periodically, by input signal, or when detecting temperature changes or focus inaccuracy in the image data.

To provide a technique capable of forming stable droplets. There is provided a microparticle sorting device including a microchip including a main flow path through which a liquid containing a microparticle flows, a sheath liquid flow path that communicates with the main flow path and through which a sheath liquid flows, and a sheath liquid introduction portion that introduces the sheath liquid, in which the sheath liquid flowing through the sheath liquid introduction portion is vibrated. Furthermore, there is also provided a microparticle sorting method including, in a microchip including at least a main flow path through which a liquid containing a microparticle flows, a sheath liquid flow path that communicates with the main flow path and through which a sheath liquid flows, and a sheath liquid introduction portion that introduces the sheath liquid, vibrating the sheath liquid flowing through the sheath liquid introduction portion.