The authors have studied photoluminescence (PL) of Mg2SnO4:Mn, Zn2SnO4:Mn, and their solid solutions Zn2xMg2(1-x)SnO4:Mn synthesized under irradiation with the millimeter wave of 28 GHz, in order to find its advantage in the synthesis of stannate phosphors. The PL spectra of Mg2SnO4:Mn exhibit a single band peaking at 500 nm. Comparison on the intensity of the 500 nm band was done between the samples synthesized by the millimeter wave heating and electric furnace heating. The origin of the difference in intensity between them will be discussed on the basis of experimental results. The influence of Zn2+ addition on PL spectra has also been investigated in Zn2xMg2(1-x)SnO4:Mn, in order to develop a new phosphor for electron beam excitation. It will be cleared whether Zn2+ addition is effective for the PL enhancement of the 500 nm band.