Enhancement of emission intensity of rare-earth ion-doped SrTiO3 by Al addition has been investigated. In Pr3+ and Tb3+, addition of 23 mol % Al intensifies emission by more than 200 times. On the other hand, in other rare-earth ions, the addition of 20 mol % Al intensifies emission at most by three times. This observation implies that 4f-5d transition plays an important role in the emission enhancement. In SrTiO3 doped with a rare-earth ion other than Pr3+ or Tb3+, a photoluminescence (PL) spectrum is changed, probably due to a decrease in the effective concentration of the ion in SrTiO3. The temperature dependence of PL spectra shows that the energy transfer from carriers to Pr3+ or Tb3+ ions is much more efficient than that to other rare-earth ions in SrTio(3). It can be speculated that the energy transfer in SrTiO3:Pr3+ or Tb3+ occurs from carriers to Pr3+ or Tb3+ ion via 4f-5d transitions, which are much higher in oscillator strength than 4f-4f transitions.