The purpose of this study is to conduct a randomised controlled trial (RCT) comparing UFT/LV and UFT/LV + PSK in patients with histological stage IIIa/IIIb colorectal cancer who have undergone curative surgery without residual cancer using 3-year disease free survival (DFS) as the primary endpoint, and also to analyze the 3-year overall survival (OS), compliance, adverse events, quality of life (QOL) and relationship with tumor factors.