A surface portion including a base material, a molecule A having a structure derived from guanidine, and a molecule B having a structure derived from at least one molecule selected from the group consisting of glucosamine and lysine, and the surface portion Is an antibacterial material that is in contact with at least a part of the substrate and has a mass ratio of the molecule A to the molecule B (molecule A: molecule B) of 8: 2 to 4: 6.基材と、グアニジンに由来する構造を有する分子Aと、グルコサミン及びリジンからなる群から選ばれる少なくとも一種の分子に由来する構造を有する分子Bと、を含む表面部と、を備え、前記表面部は前記基材の少なくとも一部と接しており、かつ、前記分子Aと前記分子Bとの質量比(分子A:分子B)が8:2~4:6である、抗菌性材料。