The base value of saturation flow rate in Japan was set in 1984. Although the base value is still used, it is considered that the base value has been fluctuating due to the changes in driver attributes and vehicle performance. Therefore, in order to clarify the variation in the base value, the saturation flow rate, which is correspond to the base value, was observed. As a result, it was shown that the saturation flow rate was below the base value at all points of the observed in the through lane, and it was averagely 15% lower than the base value. Additionally, the factors of the saturation flow rate variation were investigated focusing on the inter-vehicle time and the occupied time. As a result, it was found that the increase in inter-vehicle time contributed to the decrease of the saturation flow rate. Furthermore, lower starting acceleration and wider vehicle gap in queue waiting compared to the past were found and these driving behavior changes might cause a decrease in saturation flow rate.