An adsorbent which enables solid phase extraction of water-soluble molecules with high efficiency and excellent selectivity and an analysis system using the adsorbent, the adsorbent containing a structure represented by the formula I wherein R is a carrier component, the moiety other than R is a side-chain functional group, R and the benzene ring in the side-chain functional group are bonded directly or bonded through one or more atoms, R′ is selected from the group consisting of hydroxy group, alkoxy group, amino group, alkylamino group, thiol group and alkyl sulfide group, R″ is independently selected from the group consisting of hydroxy group, alkoxy group, alkyl group, amino group, alkylamino group, dialkylamino group, trialkylamino group, thiol group, alkyl sulfide group and hydrogen atom, x is an integer of zero or more and three or less, and n is the number of the side-chain functional groups contained in the carrier component.