Disclosed is a method capable of inexpensively forming a gradient-hardened rigid layer which has characteristics of functionally graded material on the surface layer of titanium. The method includes (a) injecting titanium into a heat treatment apparatus and performing ventilation to maintain an atmospheric pressure of 10−4 torr or less, (b) performing a pretreatment process of heating the titanium at 730 to 800° C. for 10 minutes to 5 hours to remove an oxide film formed on the surface of the titanium, (c) injecting one or more gases selected from nitrogen, oxygen, and carbon into the heat treatment apparatus and heating the titanium at 740 to 950° C. for 30 minutes to 20 hours such that a gradient-hardened rigid layer having a concentration gradient of the gases is formed on the surface of the titanium, and (d) cooling the titanium.