An analytical system comprising: a fluid analyzing apparatus having an optical detection system comprising an irradiation section and a detection section; a microchip having a main channel permitting a sheath liquid and a sample liquid to flow therethrough, wherein the microchip is mounted in the fluid analyzing apparatus, the microchip is adapted to feed liquid in a condition where a laminar flow of the sample liquid introduced by a microtube into the main channel is surrounded by a laminar flow of the sheath liquid, wherein the microtube is formed from a metal permitting a voltage to be impressed thereon, and an electric charge can be imparted to said sample liquid flowing inside said microtube, the main channel includes a narrow-down section where a bottom surface and/or an upper surface of the channel is formed as an inclined surface so that the area of the channel in a section orthogonal to a liquid feed direction decreases gradually along the liquid feed direction and where side walls of the channel are formed to exhibit gradual constriction along the liquid feed direction, the narrow down section is adapted to reduce and narrow down the sheath liquid laminar flow and the sample liquid laminar flow while being deflected toward the upper and/or the lower side of the microchip, at least two outlets are in fluid communication with the main channel through two branch channels (11a, 11b), the branch channels (11a, 11b) are branched at a branching section on the downstream side of the narrow-down section, and the microchip is adapted to selectively feed the particulates contained in the sample liquid into one of the branch channels (11a, 11b), characterized in that the microchip further comprises a fluid introduction section (91, 92) communicating with the main channel at a joining section, the joining section being downstream of the narrow down section, a booster pump adapted to supply a fluid into the fluid introduction section (91, 92), an irradiated and detected channel section on the upstream side of the joining section, the fluid analyzing apparatus is adapted to irradiate the irradiated and detected channel section by the irradiation section and to detect the irradiated and detected channel section by the detection section to detect the particulates passing through the main channel upstream of the joining section, a further optical detection system (104, 105) for determining optical characteristics of the particulates on the downstream side of the joining section. a fluid analyzing apparatus having an optical detection system comprising an irradiation section and a detection section; a microchip having a main channel permitting a sheath liquid and a sample liquid to flow therethrough, wherein the microchip is mounted in the fluid analyzing apparatus, the microchip is adapted to feed liquid in a condition where a laminar flow of the sample liquid introduced by a microtube into the main channel is surrounded by a laminar flow of the sheath liquid, wherein the microtube is formed from a metal permitting a voltage to be impressed thereon, and an electric charge can be imparted to said sample liquid flowing inside said microtube, the main channel includes a narrow-down section where a bottom surface and/or an upper surface of the channel is formed as an inclined surface so that the area of the channel in a section orthogonal to a liquid feed direction decreases gradually along the liquid feed direction and where side walls of the channel are formed to exhibit gradual constriction along the liquid feed direction, the narrow down section is adapted to reduce and narrow down the sheath liquid laminar flow and the sample liquid laminar flow while being deflected toward the upper and/or the lower side of the microchip, at least two outlets are in fluid communication with the main channel through two branch channels (11a, 11b), the branch channels (11a, 11b) are branched at a branching section on the downstream side of the narrow-down section, and the microchip is adapted to selectively feed the particulates contained in the sample liquid into one of the branch channels (11a, 11b), characterized in that the microchip further comprises a fluid introduction section (91, 92) communicating with the main channel at a joining section, the joining section being downstream of the narrow down section, a booster pump adapted to supply a fluid into the fluid introduction section (91, 92), an irradiated and detected channel section on the upstream side of the joining section, the fluid analyzing apparatus is adapted to irradiate the irradiated and detected channel section by the irradiation section and to detect the irradiated and detected channel section by the detection section to detect the particulates passing through the main channel upstream of the joining section, a further optical detection system (104, 105) for determining optical characteristics of the particulates on the downstream side of the joining section.