To clarify the anomalous behavior of magnetization of Zr₂Co₁₂P₇, we synthesized solid solution systems Zr_2−xNb_xCo₁₂P₇ and Zr_2−yY_yCo₁₂P₇ and studied their magnetism. In addition to x = 2 and y = 2, we successfully obtained polycrystalline samples in a single phase in 0 < x < 1 and 0 < y < 1. The Y-substitution did not affect the Curie temperature (T_C), while the Nb-substitution decreased T_C from 150 K to 30 K. The electron doping from Nb or local deformation may cause the decrease of T_C. The low-temperature plateau in the temperature dependence of the magnetization expands to a higher-temperature region by the Nb-substitution. A comparison of the simulation in the two-magnetic-sublattice model and experimental results showed that the magnetic moment of Nb coupled to the ferromagnetic internal field stronger than that of Zr. Qualitative agreement between the simulation and the experiment indicated that d electrons of X are localized, and their magnetic moment couples to the ferromagnetic internal field as the case of 4f electrons of lanthanoids.