A mixed liquid passage (31) is connected to a trapping passage (22) at a point upstream of a trap column (23). A mixture of a diluting liquid and an additive agent, which are respectively drawn and sent by pumps (35) and (36), is supplied into the mixed liquid passage (31). When an eluate containing a target compound is passed through the trap column (23), the eluate is diluted with the diluting liquid and hence its elution capability decreases, so that the target compound can be easily captured in the trap column (23). Meanwhile, the solubility of the mobile phase in the eluate decreases, causing not only the target component but also foreign compounds to more easily precipitate. However, their precipitation is prevented by the effect of the additive agent. Thus, clogging of the trap column (23) or pipes is prevented.