The itinerant ferromagnetism is reported in A2T12P7 (A = lanthanoid, Group 2-4 element, T = transition metal) with Zr2Fe12P7-type structure. In order to investigate the behavior of electrons in this structure, we have synthesized single crystals of (Ca1-xMgx)2Co12P7 and measured magnetizations. From magnetic susceptibility, we found that Ca2Co12P7 shows a ferromagnetic transition at TC = 78 K, which is attributed to the Co-3d itinerant electrons. We also found the Curie temperature decreases and eventually disappears around x~0.6, when Ca is substituted by Mg. It has been suggested that the electronic density of states changes and the magnetic state of this system crosses the quantum critical point due to the positive chemical-pressure effect of the Ca substitution by Mg.