Abstract: A compound which specifically binds to a receptor of NR1/NR2B, and is used as a NR2B receptor antagonist is provided. It has been found out that a piperazine derivative represented by the formula (I) binds specifically to a receptor of NR1/NR2B, and is used as a NR2B receptor antagonist. A compound represented by: wherein R1 is each independently C1-C3 alkyl or the like, m is an integer of 0 to 4, X is —N(R4)—C(?O)—C(?O)—, —N(R4)—(CR5R6)p—C(?O)—, —N(R4)—C(?O)—(CR7R8)q— or —C(?O)—N(R4)—(CR7R8)q—, p and q are each independently an integer of 1 to 3, R4, R5, R6, R7 and R8 are each independently a hydrogen atom or lower alkyl, A1 is benzoxazolinone or the like, and A2 is optionally substituted phenyl or the like, or a pharmaceutically acceptable salt or a solvate thereof.