The present invention provides a lanthanum sulfide or cerium sulfide sintered compact usable as a thermoelectric conversion material having a high Seebeck coefficient. The sintered compact has a chemical composition of La.sub.2S.sub.3 or Ce.sub.2S.sub.3, and a crystal structure consisting of a mixture of beta and gamma phases having a Seebeck coefficient higher than that of the crystal structure otherwise being in gamma single-phase. The sintered compact is produced by preparing a beta-phase La.sub.2S.sub.3 or alpha-phase Ce.sub.2S.sub.3 powder of raw material having a high purity with a suppressed carbon impurity concentration and a given range of oxygen concentration, charging the raw material into a carbon die having an inner surface covered with an h-BN applied thereon, and hot-pressing the charged material under vacuum to form a mixture of beta and gamma phases having a high Seebeck coefficient.