PURPOSE: To prevent a steel belt from deforming into a plate shape by applying surface treatment in which residual compression stress remains in a region in the vicinity of a surface on the opposite side of a roller on the opposite side of the roller of a lower steel belt before it is attached to a double belt press.

CONSTITUTION: Before a steel belt 2 is attached to a double belt press, a steel ball 48 is projected by high speed air stream toward an upper face 2" on the opposite side of a roller of a lower steel belt 2 to apply compression action to the steel belt 2 locally, causing a residual stress condition VE in a region in the vicinity of this face 2". When the steel belt 2 surface-treated in this way is attached to the double press belt and operation is done, the roller rolls sequentially on a lower face 2' of the double press belt 2 to overlap the residual stress condition occuring in the vicinity of the lower face 2' on the residual stress condition VE generated artificially on the upper face 2". Consequently, the residual stress condition E' is formed substantially symmetrically on a central face of the steel belt 2 to prevent the steel belt 2 from being curved.