The present disclosure relates to a titanium or titanium alloy member and to a surface hardening method for the titanium or titanium alloy member. The titanium or titanium alloy member includes a base material of titanium or titanium alloy, and at a surface of the base material, a hardened layer formed by diffusion of oxygen into the surface. The method includes: a heating step of heating the titanium or titanium alloy base material of the member to a predetermined temperature under an inert gas atmosphere; a hardening step of introducing (i) a mixed gas including an inert gas, and (ii) oxygen gas as a hardening treatment gas, to perform hardening treatment of the surface of the base material; and a cooling step of cooling the base material down to room temperature under the inert gas atmosphere.