PURPOSE: To stabilize surely a base line by passing the confluent liquid of plural liquids having different compsns. to a tube contg. a fine wire-shaped element and a part holding a flexible or granular material in the stage of liquid chromatography or flow injection analysis.

CONSTITUTION: Eluates are fed respectively from storage parts 1,1' for eluates having different compsns. by pumps 2, 2'. The eluate from the part 1 is mixed with a sample in a sample injector 3 and the mixture passes through a tube 4 and is joined with the eluate 1' in a confluent joint 6. The confluent liquid passes through a "Teflon" coil-shaped tube 7 contg. a fine wire-shaped element such as nylon yarn so that the diffusion in the radiation direction of the flow is made uniform. The liquid flows the thereafter through a part 8 of a filter holding flexible "Teflon" wool, etc. or a part 8 of a filter, etc. consisting of the "Teflon" wool itself so that the diffusion in the radiation direction perpendicular to the flow is made uniform. The filtered liquid is passed to a detector 9. The stable measuring base line is thus obtd.