We examined chemical changes in oysters Crassostrea gigas and packing water that were sold after storage at 5, 10, and $20^{circ}C$. The pH of oysters stored at $5^{circ}C$ dropped to 5.81 after 10 days of storage, while that of oysters at $10^{circ}C$ and $20^{circ}C$ dropped to 5.37 after 8 days and to 5.04 after 4 days, respectively. The glycogen content of oysters stored at $5^{circ}C$ decreased from 718.89 to 421.85 mg/100g during storage, while that of oysters at $10^{circ}C$ decreased to 351.49 mg/100 g after 4 days. The turbidity and soluble protein in packing water increased slightly. The viable cell count of oysters did not exceed 6 log CFU/g after 10 days of storage at $5^{circ}C$, but that of oysters at $10^{circ}C$ did so after 8 days. Additionally, the viable cell count of packing water was lower than that of oysters. We performed a principal component analysis, where the first principal component (55.03%-57.24%) and second principal component (42.76%-44.97%) described most variation. The first principal component included the pH of oysters and packing water, and the glycogen content of oysters. A Pearson correlation between the first two principal components had a higher R value than that between other components. Freshness was evaluated using the pH of oysters and packing water, and glycogen. We found that soluble protein content was significantly associated with a lower pH and glycogen content.