When human antibody genes were expressed in the methylotrophic yeast Ogataea minuta, the secreted antibody became partially degraded. To suppress the degradation, a vacuolar protease-deficient strain was constructed and its antibody production was evaluated. Although antibody productivity was improved in the vacuolar protease-deficient strain, the secreted antibody still became partially degraded. Peptide sequencing revealed that the cleavage occurred in the CH1 region of the heavy chain, implying that the cleavage was caused by an aspartic protease, Yps1p. To inhibit this cleavage, Yps1p-deficient strains were constructed and their antibody production was evaluated. As a result, the partial degradation of the antibody was suppressed in the O. minuta multiple-protease-deficient strains.