The invention relates to compounds of a general formula (I): wherein Ar1 is an optionally-substituted aryl or heteroaromatic group; R1 is an optionally-substituted lower alkyl, lower alkenyl, lower alkynyl or cyclo-lower alkyl group, or is an aryl, aralkyl or heteroaromatic group optionally having a substituent; R2 is a hydrogen atom, a lower alkyl group, a lower alkenyl group or a lower alkynyl group, or is an aryl, aralkyl or heteroaromatic group optionally having a substituent; R3 is a hydrogen atom or a lower alkyl group; R4 is a hydrogen atom, a halogen atom, a hydroxyl group, a lower alkyl group or a group of —N(R1k)R1m; T and U are a nitrogen atom or a methine group, etc. The compounds of the invention have excellent Weel kinase-inhibitory effect and are therefore useful in the field of medicines, especially treatment of various cancers.