The present invention relates to novel compounds of formula [I] or pharmaceutically acceptable salts thereof: The compounds of the present invention are useful in the prevention or treatment of diseases such as schizophrenia, Alzheimer's disease, cognitive impairment, dementia, anxiety disorders (e.g., generalized anxiety disorder, panic disorder, obsessive-compulsive disorder, social anxiety disorder, post-traumatic stress disorder, specific phobias, acute stress disorder), depression, drug dependence, spasm, tremor, pain, Parkinson's disease, attention deficit hyperactivity disorder, bipolar disorder, eating disorder, or sleep disorders, which is based on the glycine uptake-inhibiting action.