The origin of secondary phase formation in Ca3Ta(Ga1-xAlx)Si2O14 (CTGAS) single crystal growth from its melt is still controversial. Here we demonstrate that liquid immiscibility in bulk CTGAS melt is responsible for the secondary phase formation by examining other possible candidates, eutectic reaction and incongruent melting. The former and the latter possibilities were denied by the fact that the secondary phase formed even in the early stage of CTGAS single crystal growth using the micro-pulling down (mu-PD) method and that the differential thermal analysis (DTA) demonstrated the congruency of the CTGAS. We analyzed compositions of the secondary phases and the matrix in solidified CTGAS melt varying the ratio of Al to Ga in Ga site and temperature. The electron probe micro-analyzer (EPMA) analysis revealed that three kinds of phases, the ratios of which are equal, existed in the solidified melt and their populations varied depending on the ratio of Al to Ga. We demonstrate that the same ratio of Al to Ga between these three phases regardless of their population can be explained only by the presence of liquid immiscibility in CTGAS melt during growth.