A novel compound having histone deacetylase-inhibiting activity which is a cyclic tetrapeptide derivative represented by the general formula (1) given below and a pharmaceutical composition comprising such compound as an active ingredient. (In the formula, the cyclic tetrapeptide moiety has a known structure R1 and R2 each independently represents an alkylene group containing 1 to 6 carbon atoms, which may be branched X represents a group or bond selected from among —CO—, —O—, —S— or —SO— Y represents a hydrogen or halogen atom, a phenyl group (including a substituted form), a pyridyl group (including a substituted form), an alkyl group (including a halogen-substituted form hereinafter the same shall apply) containing 1 to 6 carbon atoms, an alkyloxy group containing 1 to 6 carbon atoms, an alkylcarbonyl group containing 1 to 6 carbon atoms, an alkyloxycarbonyl group containing 1 to 6 carbon atoms, an alkylthio group containing 1 to 6 carbon atoms, an alkylthiocarbonyl group containing 1 to 6 carbon atoms or a mono- or dialkylamino group containing 1 to 6 carbon atoms when Y is a phenyl group (including a substituted form) or a pyridyl group (including a substituted form), it may form a further cyclic structure bound to R2.)
