We developed a method to rapidly evaluate the freshness of fish using a sensor gas chromatography (SGC) system. Mackerel Scomber japonicus was stored at 4°C for 12 days to create an environment similar to the natural decomposition process. Trimethylamine (TMA) content in mackerel muscle was measured at 3-day intervals using a spectrophotometer. The gas-phase concentration of TMA in whole mackerel was also determined using an SGC system. The muscle TMA content increased over time during storage, as did the gas-phase concentration of TMA. Therefore, this study demonstrated that an SGC system can be used to rapidly measure the gas-phase concentration of TMA in fish during processing.