It is intended to provide an apparatus for LC/MS analysis to be used in analyzing oligonucleic acids. A liquid chromatographic mass spectrograph which has a pump for feeding an eluent containing an ion pairing agent and a salt, a sample injector for injecting a sample to be analyzed into a channel, a column for fractionating the injected solution depending on components, and a mass spectrograph for ionizing the solution eluted from the column by an atmospheric ion source and detecting the same, characterized by further having a recombining means of adding a weakly basic solution between the column and the mass spectrogram. Owing to the above constitution, an oligonucleic acid can be analyzed at a high efficiency and a high sensitivity.
