A sulfur-containing bicyclic compound having a PAM action of GABAB, and it use for preventing and/or treating schizophrenia, CIAS, cognitive impairment, fragile X syndrome, autism spectrum disorder, spasticity, anxiety disorder, substance addiction, pain, fibromyalgia, or Charcot-Marie-Tooth disease. A pharmaceutical composition for preventing and/or treating schizophrenia, cognitive impairment associated with schizophrenia (CIAS), cognitive impairment, fragile X syndrome, autism spectrum disorder, spasticity, anxiety disorder, substance addiction, pain, fibromyalgia, Charcot-Marie-Tooth disease, or the like, which has a positive allosteric modulator (PAM action) of GABAB.