Upon developing therapeutically potent antibodies, there are significant requirements, such as increasing their affinity, regulating their epitope, and using native target antigens. Many antibody selection systems, such as a phage display method, have been developed, but it is still difficult to fulfill these requirements at the same time. Here, we propose a novel epitope-directed antibody affinity maturation system utilizing mammalian cell survival as readout. This system is based on the competition of antibody binding, and can target membrane proteins expressed in a native form on a mammalian cell surface. Using this system, we successfully selected an affinity-matured anti-ErbB2 single-chain variable fragment variant, which had the same epitope as the original one. In addition, the affinity was increased mainly due to the decrease in the dissociation rate. This novel cell-based antibody affinity maturation system could contribute to directly obtaining therapeutically potent antibodies that are functional on the cell surface.