The present inventors achieved the present invention on the basis of the knowledge that a thiazole derivative with a pyrazine-2-carbonylamine substituent at the second carbon position is an excellent positive allosteric modulator of the muscarinic M3 receptor and is useful as a prophylactic and/or therapeutic agent against bladder/urinary tract disorders associated with bladder contractions induced by the muscarinic M3 receptor. A 2-acylaminothiazole derivative or a salt thereof according to the present invention can be used as a prophylactic and/or therapeutic agent against bladder/urinary tract disorders associated with bladder contractions induced by the muscarinic M3 receptor, including, for example, urinary disorders such as underactive bladder. (In the formula, R
