Abstract: A novel thiazolidine derivative of the formula: ##STR1## wherein R.sup.1 is a substituted or unsubstituted phenyl group, Q is a single bond, a lower alkylene group or a lower alkenylene group, R.sup.2 and R.sup.3 are the same or different and each is hydrogen atom, a lower alkyl group, a cycloalkyl group, a lower alkanoyl group, a lower alkoxycarbonyl group, a lower alkylsulfonyl group, a benzoyl group, a phenyl group or a di(lower alkyl)phosphoryl group, Alk is a lower alkylene group, and Y and Z are the same or different and each is oxygen atom or sulfur atom, or a pharmaceutically acceptable salt thereof, which is useful as a cardiotonic agent is disclosed together with processes for the preparation of the compound and a pharmaceutical composition containing said compound.