A luminous body of prolonged fluorescence lifetime characterized by comprising not only an activator but also at least one coactivator selected from the group consisting of La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Bi, Sn, Sb and analogues thereof as a further luminescent center for enhancing the thermostability of luminous body. This luminous body excels in thermostability and fluorescence lifetime, so that it is useful as a luminous body for LED.