Norovirus (NoV) is a major cause of food poisoning outbreaks in Korea. Most NoV outbreaks originate from environmental contamination, but bivalves such as oysters are also important vectors. Oyster Crassostrea gigas contamination by NoV has been reported in Korea, but no quantitative analyses of NoV have been performed. We investigated the NoV concentration in 21 oyster samples from a Korean commercial oyster-growing area with confirmed fecal contamination from January to December 2012, using real-time reverse transcription-polymerase chain reaction. Additionally, we assessed the NoV concentration after heating to investigate the effects of heat treatment on NoV-infected oysters. In NoV-positive samples, the cycle threshold (Ct) values were 37.43-39.41 and 36.77-39.30, while viral concentrations were $8.97{ imes}10^2-2.24{ imes}10^2$ and $3.05{ imes}10^2-7.47{ imes}10^1$ copies/g for genogroups I and II, respectively. After heat treatment, NoV genogroup I decreased by 83.4%, 88.0%, 89.4% and 100% at $60^{circ}C$, $68^{circ}C$, $70^{circ}C$, and $100^{circ}C$, respectively, for 15 min, while genogroup II respectively decreased by 67.3%, 76.3%, 80.1%, and 89.8% under the same conditions.