The present invention relates to a method of localizing an intact immunoglobulin-format antibody in cytosol by permeating membrane of cells. The present invention also relates to a light-chain variable region (VL) that induces an intact immunoglobulin-format antibody to penetrate the membrane of living cells and be localized in the cytosol, and to an antibody comprising the same. The present invention also relates to a biologically active molecule fused to the antibody and selected from the group consisting of peptides, proteins, small-molecule drugs, nanoparticles and liposomes. The present invention also relates to a composition for prevention, treatment or diagnosis of cancer, comprising: the antibody or a biological active molecule fused to the antibody and selected from the group consisting of peptides, proteins, small-molecule drugs, nanoparticles and liposomes. The present invention also relates to a polynucleotide that encodes the light-chain variable region and the antibody. The present invention also relates to a method for producing an antibody which penetrates cells and is localized in the cytosol.According to the method of the present invention, which allows an intact immunoglobulin-format antibody to actively penetrate living cells and be localized in the cytosol, the antibody can penetrate living cells and be localized in the cytosol, without having to use a special external protein delivery system. Moreover, the use of the cytosol-penetrating light-chain variable region according to the present invention and the intact immunoglobulin-format antibody comprising the same can penetrate cells and remain in the cytosol, without affecting the high specificity and affinity of a human antibody heavy-chain variable region (VH) for antigens, and thus can be localized in the cytosol which is currently classified as a target in disease treatment based on small-molecule drugs, and at the same time, can exhibit high effects on the treatment and diagnosis of tumor
