[Problem] To provide a compound useful as a pharmaceutical composition for prevention and/or treatment of diseases such as schizophrenia. [Solution] The inventors developed the present invention upon investigating a pharmaceutical composition having effects as a GABAB positive allosteric modulator (effects as a PAM), for preventing/treating diseases such as schizophrenia, and confirming that a sulfur-containing bicyclic compound has effects as a GABAB PAM. This sulfur-containing bicyclic compound has effects as a GABAB PAM and can be used as a preventive and/or therapeutic agent for diseases such as schizophrenia. (In the formula, X represents CH, R1 represents a lower alkyl, R2 represents a lower alkyl , R3 represents H, R4 represents H, ring A is a cyclohexane ring, RY represents a group represented by formula (III), Y represents NH, etc., and RL represents a lower alkyl.)
