The photoluminescence (PL) of a resonant-tunneling transistor was studied. An excitation energy close to the bandgap energy of the GaAs collector layer was chosen so as to excite neither the barrier layer nor the quantum well, resulting in simple luminescence spectra. The PL signal shows a strong correlation with resonant-tunneling current. The PL peak position shifts to lower energy with increasing collector voltage, V-ce indicating the existence of the quantum-confined Stark effect. The V-ce dependencies of PL intensity and line width suggests charge buildup in the quantum well. The effect of gate voltage on PL spectra suggests a quantum confinement by the potential of the gate depletion layer.