To realize a favorable cushioning properties without making a foam thicker by a method wherein triangular projections are continuously formed on the top surface of a hollow formed body, which supports the foam below.

A hollow formed body 7 is formed so as to have a closed crosssectional shape made by surrounding a hollow part 11 with a frame part 9 at a peripheral portion and a seating surface part 10 at the top surface portion inside the frame part 9. In the seating surface part 10, a plurality of triangular projections 12 are continuously formed. The projections 12 are formed so as to project upwards in triangles and, at the same time, run parallel to one another to the right and left directions of a seating. A cushioning properties are imparted by placing a foam on the hollow formed body 7. At the seating of the foam 8 on the hollow formed body 7, the valley parts of the projections 13 of the foam 8 coincide with the top parts of the projections 12 of the hollow formed body 7 and the top parts of the projections 13 of the foam 8 with the valley parts of the projections 12 of the hollow formed body.