PURPOSE: To avoid waves, etc. by forming plural lines of longitudinal grooves at the bottom of an eaves gutter core material and many lines of transverse grooves at the side walls, respectively, and applying bending work after covering it with a synthetic resin layer.

CONSTITUTION: Many holes 17 are made excepting both ends of a core material 1, consisting of a long steel plate, and corners 3b. Next, the burrs of the hole 17 are crushed with upper and lower burr crushing rolls, and then longitudinal grooves 4a, at the bottom 3a, and transverse grooves 4b, at the sidewalls 5a, are made respectively. Next, the whole face of the core material 1 is covered with a synthetic resin layer 2. And when bending it, the corner part shrinks in the longitudinal direction, whereas the longitudinal grooves 4a resist the stress accompanying the shrinkage. Accordingly, it can avoid waves or bulges from occurring at the bottom 3 and the sidewalls.