Disclosed is a means for reducing side effects of an immune checkpoint regulator that is used as an anticancer drug or the like. A side-effect reducing agent according to the present invention comprises as an effective ingredient an anti-CD4 antibody having a high cytotoxic activity, or an anti-CD4 antibody or antigen-binding fragment thereof which antibody or fragment comprises a cytotoxic component bound thereto. The anti-CD4 antibody is a human-type chimeric antibody, humanized antibody or human antibody against human CD4. The immune checkpoint regulator may be, for example, an anti-PD-L1 antibody, an antagonistic anti-CTLA-4 antibody, or an agonistic anti-OX40 antibody.
