The present invention relates to: a dihydroimidazooxazole derivative represented by general formula (IA) or a pharmaceutically acceptable salt thereof, which has positive allosteric modulator activity on metabotropic glutamate receptor subtype 2 (mGlu2 receptor); and a pharmaceutical product for prevention or treatment of a disease selected from the group consisting of schizophrenia, Alzheimer's disease, cognitive impairment, dementia, anxiety disorders, depression, attention deficit/hyperactivity disorder (AD/HD), drug dependence, convulsions, tremor and sleep disorders, said pharmaceutical product containing the dihydroimidazooxazole derivative or a pharmaceutically acceptable salt thereof as an active ingredient.本發明係關於對於代謝型谷胺酸受體亞型2(mGlu2受體)具有正變構調節劑(Positive allosteric modulators)作用之一般式(IA)所示新穎之二氫咪唑並噁唑衍生物、或其醫藥上可被許可的鹽。其可作為精神分裂症、阿茲海默症病、認知功能障礙、痴呆、焦慮障礙、鬱悶病、AD/HD(缺乏關注/多動性障礙)、藥物依存、痙攣、發抖、睡眠障礙等疾病的預防或治療用醫藥時為有用。