Provided is a plastic surface treatment method, which is a Cr-free plastic surface treatment method that is capable of applying a plating on a plastic surface that adheres sufficiently closely thereto. The method is a plastic surface treatment method characterized in that the plastic is treated with a solution in which sulfuric acid has been electrolyzed. The sulfuric acid concentration of the sulfuric acid solution is 50-92 wt%. The persulfuric acid concentration is at least 3 g/L. It is preferable that the treatment temperature is at least 80 DEG C, for example, 80-140 DEG C and particularly100-130 DEG C. By immersing the plastic in said sulfuric acid solution for 1-10 minutes, hydrophilic functional groups are exposed on the surface of the plastic.