A method for surface treatment capable of easily improving a mechanical strength of an internal combustion piston at a reasonable cost is provided. A modified surface layer is formed by injecting injection powders containing a reinforcing element to be collided with an Al—Si alloy-based piston obtained by casting and forging by injecting under predetermined conditions, the reinforcing element being diffused and penetrated in the piston to improve the strength thereof. When a function, such as fuel modification, is imparted to the modified surface layer, an element exhibiting a photocatalytic function by oxidation, such as Ti, Sn, Zn, Zr, or W, is selected as the reinforcing element. By locally heating and cooling performed on the piston surface by the collision with the injection powders, alloy elements are fine-grained by recrystallization, the reinforcing element in the injection powders is diffused and penetrated in the piston surface by activated adsorption, and a modified layer having a uniformly fine-grained microstructure containing the alloy elements and the reinforcing element is formed. As a result, besides improvement in strength of the piston, by the selection of the above element, such as Ti, the photocatalytic function, such as fuel modification, can also be obtained.