To provide a surface processing method of aluminum casting product capable of removing a casting defect due to friction stir processing without damaging an effect imparted by heat treatment such as improvement of fatigue strength and without bringing about elongation of processing time.

In the surface processing method of aluminum casting product, an aluminum casting product is made to be an object to be processed, and the prescribed heat treatment and the friction stir processing for reforming a surface part of the object to be processed by stirring in the non-fusion state caused by frictional heat, by inserting the prescribed rotation tool to the surface part of the object to be processed while rotating the prescribed rotation tool are performed. Therein, a solution heat treatment (S20) and an aging treatment (S40) are performed as the heat treatment, and the friction stir processing (S30) is performed between the solution heat treatment (S20) and the aging treatment (S40).