A pharmaceutical composition for use in a method for treating or preventing thrombocytopenia, the composition comprising a compound derived from 2-acylaminothiazole represented by the following Formula (I), or a pharmaceutically acceptable salt thereof, as an active ingredient: wherein the symbols have the following meanings: Ar1: aryl, monocyclic aromatic heterocycle or optionally substituted bicyclic condensed heterocycle, with the proviso that when R1 is aryl or pyridyl, each of which may be substituted with one or more groups selected from the group that consists of lower alkyl, -CO-lower alkyl, -COO-lower alkyl, -OH, -O-lower alkyl, -OCO-lower alkyl and halogen, Ar1 is not phenyl or pyridyl, each of which may be substituted with one or more groups selected from the group consisting of lower alkyl, -CO-lower alkyl, -COO-lower alkyl, -OH, -O-lower alkyl, -OCO-lower alkyl r and halogen； R1: monocyclic aromatic aryl or heterocycle, each of which may be substituted, R2: a group represented by the following General formula (II): ** Formula ** n: an integer from 1 to 3； m: an integer from 1 to 3； where when n or m is an integer of 2 or more, CR20R21 and CR22R23 may be identical or different； X: O, S, or a group represented by N-R26 or C (-R27) -R28, R20, R21, R22, R23, R26, R27 and R28: which may be -H； -OH, -O-lower alkyl； optionally substituted lower alkyl； optionally substituted cycloalkyl； optionally substituted aryl； optionally substituted arylalkyl； optionally substituted aromatic heterocycle； optionally substituted aromatic heterocyclic alkyl； optionally substituted non-aromatic heterocycle； optionally substituted lower alkenyl； optionally substituted lower alkylidene； -COOH； -COO-lower alkyl； -COO-lower alkenyl； -COO- (lower alkylene) -aryl； -COO- (lower alkylene)-aromatic heterocycle； identical or different carbamoyl or amino, each of which may be substituted with one or more groups selected from the group consisting of lower alkyl and cycloalkyl, each of whi
