A thiazole derivative of the formula (I): wherein R is a 1-optionally substituted-6-oxo-1,6-dihydro-3-pyridazinyl, R′ is an optionally substituted phenyl, and R2 is hydrogen, a group of the formula (i): wherein R4 is hydrogen, lower alkyl or lower alkenyl, and R5 is hydrogen, optionally substituted lower alkyl, acyl, cyclo(lower)alkyl, lower alkenyl, optionally substituted aryl or heterocyclic, or a group of the formula (ii): wherein X is oxygen or sulfur, R8 is hydrogen or lower alkyl, R9 is hydrogen, optionally substituted lower alkyl, cyclo(lower)alkyl, lower alkoxy or mono- or di-lower alkylamino or R8 and R9 may combine together to form optionally substituted saturated N-containing heterocyclic, or a salt thereof.