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We studied a solid-solution system of the transition-metal pnictide, Y₂(MnxFe₁−x)₁₂P₇, with the Zr₂Fe₁₂P₇-type structure, which has two kinds of transition metal sites: three tetrahedral sites and a pyramidal site. We successfully obtained polycrystalline samples of Y₂(MnxFe₁−x)₁₂P₇ in the range of 0 ≦ x ≦ 0.9. The lattice parameter along the a-axis monotonically increases with the Mn substitution, while that along the c-axis is nearly constant. ⁵⁷Fe Mössbauer spectroscopy shows that Mn atoms prefer to occupy the pyramidal site. At x = 0.25, the site-ordered compound, Y₂Mn₃Fe₉P₇, was obtained, where Mn atoms occupy the pyramidal site and Fe atoms occupy the tetrahedral sites. The magnetization of Y₂Mn₃Fe₉P₇ exhibits an anomaly at approximately 60 K, suggesting presence of an antiferromagnetic transition, which originates from the magnetic moment of Mn at the pyramidal site.