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, R1is 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.