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, R1 is -(N(-R11)(-R12) or an optionally substituted cyclic amine； R11 is a C16 alkyl； R12 is an optionally substituted C16 alkyl or an optionally substituted C38 cycloalkyl； R2 is an optionally substituted aryl, an optionally substituted monocyclic aromatic heterocycle, or an optionally substituted bicyclic aromatic heterocycle； and R3 is -H,-OH, -O-( C16 alkyl), or a halogen.)
