and having a pressure groove 7 f corresponding to the pressure frame 7 d upon implementing the pressing procedure the flat structures 6 have the same dimension as an inner area surrounded by the rectangular frame 7 k and placed between the upper press die 7 a and the lower press die 7 b as implemented in the first embodiment of the invention as shown in fig4 then during the procedure in which the upper press die 7 a is moved toward the lower press die 7 b the rectangular frame 7 k is forced to penetrate into the rectangular groove 7 e and while at the same time depressing the pressure frame 7 d against the flat structure 6 to penetrate the flat structure 6 into the pressure groove 7 f after completing the pressing procedure the upper press die 7 a is lifted to withdraw the rectangular frame 7 k from the rectangular groove 7 e this procedure imparts a cosmetic surface k with the upper surface side 7 b of the flat structure 6 and while at the same time adding an strengthened jut j with the lower surface side 7 a of the flat structure 6 as shown in fig1 and 18 as a fourth embodiment of the invention two flat structures 6 are prepared in order to undergo the injection to mold the synthetic resin body 11 as shown in fig1 through 21 with the concave recess provided on the flat structures 6 to serve as the strengthened jut j it is possible to significantly reinforce the synthetic resin body 11 with the minimum cost as a fifth embodiment.