Mn1-xTxGe4 (T: Co, Fe) solid solution was synthesized at 6 GPa and 650-750°C using a belt-type high-pressure apparatus. The crystal structure of the Mn1-xTxGe4 solid solution is related to the β-NiHg4 type. The following structural changes were observed in the Mn1-xCoxGe4 system: tetragonal (x = 0) to orthorhombic (0 < x < 0.2) and orthorhombic to pseudotetragonal (0.2 ≦ x ≦ 0.4). Similar structural changes were also observed at 0 < x < 0.4 (tetra. → ortho.) and x = 0.4 (ortho. → pseudotetra.) in the Mn1-xFexGe4 system. These results indicate that the structural changes in Mn1-xTxGe4 strongly depend on the average valence electron number of the transition metal atoms. Mn1-xTxGe4 solid solutions are itinerant electron ferromagnets and the Curie temperature decreases linearly with decreasing unit-cell volume.