It is intended to provide a microchannel for separating blood plasma whereby blood plasma can be conveniently separated on a microchip.A microchannel comprising: a substrate body (10a); a separation channel (11) which is formed within the substrate body (10a) and extended in the direction parallel to the longitudinal direction of the substrate body (10a); at least two (first and second) connection ports (13a, 14a) which are formed at one end of the separation channel (11) close to the thickness direction perpendicular to the longitudinal direction of the substrate body (10a); and first and second branch channels (13, 14) which are formed within the substrate body (10a) and connected respectively to the first and second connection ports (13a, 14a). In the state where the substrate body (10a) is extended perpendicular to the direction of gravitational force, a blood sample containing blood cells and plasma is flown from one end (11a) side of the separation channel (11). Before the blood sample attains the other end (11b) of the separation channel (11), the blood cells in the blood sample settle out. The blood cells having settled out are then introduced into the branch channel (14) that is located lower in the direction of gravitational force between the first and second branch channels. On the other hand, the major portion of the blood plasma is introduced into the other branch channel (13) located upper in the direction of gravitational force.