The present invention is to provide an aliphatic nitrogen-containing 5-membered ring compound represented by the formula [I]: wherein symbols in the formula have the following meanings; A: —CH2— or —S—, B: CH or N, R1: H, a lower alkyl group, etc., X: a single bonding arm, —CO—, -Alk-CO—, —COCH2—, -Alk-O—, —O—CH2—, —SO2—, —S—, —COO—, —CON(R3)—, -Alk-CON(R3)—, —CON(R3)CH2—, —NHCH2—, etc., R3: hydrogen atom or a lower alkyl group, Alk: a lower alkylene group, and R2: (1) a cyclic group which may be substituted, (2) a substituted amino group, etc., provided that when X is —CO—, then B is N, or a pharmaceutically acceptable salt thereof.