To facilitate the construction by laying a grooved Styrofoam after placing mat slab concrete, laying an electromagnetic shielding panel over the Styrofoam to form an electromagnetic shielding layer.

Mat slab concrete 1 is placed, and a grooved Styrofoam 2 is laid on the mat slab concrete 1. Further, a steel plate 3 as electromagnetic shielding material is laid over the grooved Styrofoam 2, and cinder concrete 4 is placed on the steel plate 3 to form an electromagnetic shielding layer. Since the top end of the groove Styrofoam 2 is flat, the construction can be simplified only by laying the steel plate 3, and the steel plate 3 can prevents the deterioration of electromagnetic shielding performance by penetration of underground water. Accordingly, an electromagnetic shielding layer excellent in durability can be formed.