Thiazole derivatives having pyrazine-2-carbonylamino substituted at the 2-position of the following formula (I): are excellent muscarinic M3 receptor positive allosteric modulators, and are useful for treating bladder or urinary tract diseases, related to bladder contraction by a muscarinic M3 receptor. The 2-acylaminothiazole derivatives and salts thereof can be used for treating bladder or urinary tract diseases, related to bladder contraction by a muscarinic M3 receptor, for example, voiding dysfunctions such as underactive bladder.