A heterocyclic compound represented by the general formula (1) or a salt thereof: wherein m, l, and n respectively represent an integer of 1 or 2; X represents —O— or —CH2—; R1 represents hydrogen, a lower alkyl group, a hydroxy-lower alkyl group, a protecting group, or a tri-lower alkylsilyloxy-lower alkyl group; R2 and R3, which are the same or different, each independently represent hydrogen or a lower alkyl group; or R2 and R3 are bonded to form a cyclo-C3-C8 alkyl group; and R4 represents an aromatic group or a heterocyclic group, wherein the aromatic or heterocyclic group may have one or more arbitrary substituent(s).