Provided are a fluorescent substance represented by the general formula (A1-xRxM2X)m(M2X4)n, a process for producing the fluorescent substance, and a luminescent device including the fluorescent substance. (In the formula, element A is one or more elements selected from Li, Na, Be, Mg, Ca, Sr, Ba, Sc, Y, La, Gd, and Lu; element R is one or more elements selected from Mn, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb; element M is one or more elements selected from Si, Ge, Sn, Ti, Hf, Zr, Be, B, Al, Ga, In, Tl, and Zn; element X is
oxygen and / or
nitrogen; n and m each is an integer of 1 or larger; and x is a real number of 0 < x <1.)