A proton pump inhibitor containing a compound represented by the formula (I) wherein X and Y are the same or different and each is a bond or a spacer having 1 to 20 carbon atoms in the main chain, R1 is an optionally substituted hydrocarbon group or an optionally substituted heterocyclic group, R2, R3 and R4 are the same or different and each is a hydrogen atom, an optionally substituted hydrocarbon group, an optionally substituted thienyl group, an optionally substituted benzo[b]thienyl group, an optionally substituted furyl group, an optionally substituted pyridyl group, an optionally substituted pyrazolyl group, an optionally substituted pyrimidinyl group, an acyl group, a halogen atom, a cyano group or a nitro group, R5 and R6 are the same or different and each is a hydrogen atom or an optionally substituted hydrocarbon group, which has a superior proton pump action and shows an antiulcer activity and the like after conversion to a proton pump inhibitor in the body, or a salt thereof, or a prodrug thereof is provided.