An inlet (37a) of a capillary tube channel that feeds a liquid from a mixing cavity (39) to measurement spots is formed near one (4) of the wall surfaces of the upper and lower surfaces of the mixing cavity (39) situated in a direction of oscillation for mixing. On the other wall surface (3), a level difference (39a) is formed such that an inner gap is larger than an outer gap in the mixing cavity (39), so that a solution is reliably fed from the mixing cavity to the measurement cells.