A liquid chromatograph having a flow passage switching mechanism for switching among a normal analysis flow passage formed by connecting a mobile-phase-sending-unit, a second mixer having a large capacity, a sample injection portion, an analysis column and a detector in this order, a concentration flow passage formed by connecting a concentration liquid sending unit for sending a liquid for transferring a sample to be concentrated, the second mixer, the sample injection portion and a trapping column in this order, and a concentration analysis flow passage formed by connecting the mobile-phase-sending-unit, a first mixer having a smaller capacity than the second mixer, the trapping column, the analysis column and the detector in this order so that any one of the three flow passages is selectively formed, The mobile-phase-sending-unit is set so that the flow rate of a mobile phase becomes smaller when the concentration analysis flow passage is selected than when the normal analysis flow passage is selected.