We have developed a manufacturing system by combination of high-pressure synthesis method using a multi-anvil press, and spark plasma sintering (SPS) method. By means of the system, we have succeeded in synthesizing new filled skutterudite-type thermoelectric materials MmxCo4Sb12 (Mm=mischmetal). The thermoelectric properties of partially filled skutterudite compounds MmxCo4Sb12 synthesized under high pressure have been investigated. The Seebeck coefficient of MmxCo4Sb12 shows negative value, which means n-type conductivity. The highest dimensionless figure of merit ZT value is 0.25 for Mm0.6Co4Sb12 at 700 K.