The present invention aims to provide novel compounds of formula [I] or pharmaceutically acceptable salts thereof that are based on a glycine uptake inhibiting action and which are useful in the prevention or treatment of such diseases as schizophrenia, Alzheimers disease, cognitive dysfunction, dementia, anxiety disorders (generalized anxiety disorder, panic disorder, obsessive-compulsory disorder, social anxiety disorder, posttraumatic stress disorder, specific phobia, acute stress disorder, etc.), depression, drug addiction, spasm, tremor, and sleep disorder:
