Excitonic absorption was observed in a transmittance spectrum of AlGaN/GaN/AlGaN single quantum well structure with a well width of 5 nm at room temperature. The total internal electric field strength in the well was about 0.73 MV/cm, which was estimated from the absorption peak position based on a simple calculation, neglecting excitons. The observation is clearly due to the quantum-confined Stark effect. While excitonic absorption was clearly observed even in such a high internal field, no light emission was detected under a He-Cd laser excitation at temperatures ranging from room temperature to T = 10 K. Light emission accompanied by a blue shift of the emission peak and an increase of emission intensity was observed under higher excitation power density. The obvious conclusion in the present case is that the presence of a high internal electric field in the well is a disadvantage for light emission.