Disclosed are a novel therapeutic means effective and practical against cancer, and a novel substance useful as such a therapeutic means. Provided are novel peptides derived from a partial region of HMGN1, HMGN2, HMGN4 or HMGN5, and anti-cancer agents and anti-cancer effect enhancers containing the peptide as an active ingredient. The peptide of the present invention has an anti-tumor effect even independently, and exerts a remarkably excellent anti-tumor effect particularly when used in combination with an immune checkpoint regulator, or an anti-CD4 antibody or antigen-binding fragment thereof.