PURPOSE:To enable detection of trace components even at a high pressure and a high flow rate, by transmitting an ultrasonic wave to an eluate from a column of a liquid chromatograph to receive the reflected wave from the liquid. CONSTITUTION:A sample is injected into an eluate and flows into a column of a liquid chromatograph to separate desired components with the column, the eluate from which is fed to an inflow port 11 of an ultrasonic detector 16 and reaches an ultrasonic detector section 13. Ultrasonic pulse is transmitted to the eluate from an ultrasonic detector 14 to be reflected with a reflection wall 15 as opposed thereto and the reflected wave thereof is received again with the ultrasonic detector 14. The reception pulse thereof is converted into an output signal with a converter to obtain a chromatogram. Thus, the eluate flows between the ultrasonic detector and the reflection wall facing each other thereby enabling highly accurate detection of trace components depending on a difference in the propagation rate of the eluate.