A compound represented by the following formula (I) or a pharmaceutically acceptable salt thereof works as an mGluR2 antagonist, and is applicable as a therapeutic agent for neurological disorders related to glutamate dysfunction and diseases involving the mGluR2, such as Alzheimer's disease: wherein R is a hydrogen atom, a C1-6 alkyl group or the like, R1 is a C1-6 alkyl group, a C1-6 alkoxy group or the like, R2 is a halogen atom, a C1-6 alkyl group, a C1-6 alkoxy group or the like, R3 is a hydrogen atom, a C1-6 alkyl group or the like, and R4 is a C1-6 alkyl group or the like.