A microchannel has a substrate body; a separation channel which is formed within the substrate; at least two (first and second) connection ports which are formed at one end of the separation channel close to the thickness direction perpendicular to the longitudinal direction of the substrate body; and first and second branch channels connected respectively to the first and second connection ports. When the substrate body is perpendicular to gravity, a blood sample containing blood cells and plasma, flows from one end of the separation channel. Before the blood sample gets to the other end of the separation channel, the blood cells in the blood sample settle out. The blood cells having settled out are then separated into the branch channel.