Berry fruits that contain large amounts of polyphenol compounds are expected to exhibit health and anti-aging effects due to the antioxidant activities of these components. Among the various polyphenols, flavan-3-ol derivatives are known to have a particularly high functionality. In this study, the maturity of red raspberry fruits is classified into eight stages based on the polyphenol content at each stage. Quantification of the various compounds and investigation of the DPPH and ABTS radical scavenging activities were carried out. The total polyphenol content, including that of the flavan-3-ol derivatives, was the highest in immature fruits, gradually decreasing during fruit maturation, during which the radical scavenging activity also decreased. Based on our quantitative results, it was considered that the decrease in the flavan-3-ol derivative content due to fruit ripening was largely related to the increase in the amount of anthocyanin derivatives. Considering that the decreased contents of these compounds were related to the expression levels of polyphenol biosynthetic enzymes, quantification was performed using the semi-quantitative polymerase chain reaction, but the only change observed was the increased expression of the enzyme that synthesizes anthocyanins during maturation. Therefore, it was suggested that it is necessary to inhibit anthocyanin synthesis to increase the contents of highly functional flavan-3-ol derivatives in the mature fruit.