The present invention provides a compound having a superior acid secretion inhibitory action, an antiulcer activity and the like.A proton pump inhibitor containing a compound represented by the formula (I) wherein ring A is a saturated or unsaturated 5- or 6-membered ring group optionally having, as a ring-constituting atom besides carbon atom, 1 to 4 hetero atoms selected from a nitrogen atom, an oxygen atom and a sulfur atom, ring-constituting atoms X1 and X2 are each a carbon atom or a nitrogen atom, a ring-constituting atom X3 is a carbon atom, a nitrogen atom, an oxygen atom or a sulfur atom, R1 is an optionally substituted aryl group or an optionally substituted heteroaryl group, R2 is an optionally substituted alkyl group, an optionally substituted aryl group or an optionally substituted heteroaryl group, R3 is an aminomethyl group optionally substituted by 1 or 2 lower alkyl groups, which is a substituent on a ring-constituting atom other than X1, X2 and X3, and ring A optionally further has substituent(s) selected from a lower alkyl group, a halogen atom, a cyano group and an oxo group, or a salt thereof or a prodrug thereof.