Ca3NbGa3Si2O14 (CNGS) is one of the langasite-type crystals and it is known as a piezoelectric material. We have successfully grown the shaped CNGS crystal using micro-pulling-down (μ-PD) method with shape control. However, the impurity phases such as Ca-Nb-O related compounds were detected in the grown crystals. The creation of the impurity phases is related to difference in stoichiometric and congruent compositions. Therefore, the detailed investigation of phase diagram on CNGS was carried out to prevent the impurity phases creation. It follows from the TG-DTA measurements that the melting point systematically changed with the Ca/Ga ratio in the Ca3-xNbGa3+xSi2O 14±δ cmpositions. The CNGS phase with Ca/Ga = 1.02 indicated the maximum melting point which suggests that the stoichiometric composition of CNGS is different in the congruent composition. Furthermore, in the DTA measurements the peaks of impurity phases were detected around 1550 °C for the CNGS sample with Ca/Ga = 1.02.