Abstract: Disclosed are a thiazole derivative represented by the following formula, a pharmaceutically acceptable salt thereof and leukotriene antagonist containing the same as the active ingredients: ##STR1## wherein R.sub.1 and R.sub.2 each independently represent a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, a lower alkoxycarbonyl group or a substituted or unsubstituted phenyl group or cooperatively represent a tetramethylene group corresponding to a fused cyclohexane ring or a butadienylene group which is unsubstituted or substituted with a halogen atom, a lower alkoxy group, a lower alkoxycarbonyl group or an alkyl group having 1 to 3 carbon atoms corresponding to a fused benzene ring; R.sub.3, R.sub.4, R.sub.5 and R.sub.