[Problem] Provided is a compound which is useful as 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. [Means for Solution] The present inventors have studied that 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 GABA B , and have found that a sulfur-containing bicyclic compound is a PAM of a GABA B receptor, thereby completing the present invention. The sulfur-containing bicyclic compound of the present invention has a PAM action of GABA B , and can be used as an agent 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.