the solder bumps are formed to the package board alternatively when the redistribution board 4 is mounted by the au u2014 sn bonding au stud bumps are formed to the electrode pads of the package board 2 and tin plating process is performed to the electrode pads of the redistribution board 4 next in step s 29 as shown in fig6 a the redistribution board 4 is mounted on the package board 2 in step s 30 the under fill material 46 is injected between the redistribution board 4 and the package board 2 thereafter in step s 31 as shown in fig6 b the base board 20 is removed the base board 20 is removed by the same method as the above described first embodiment as described above in this embodiment the redistribution board 4 is mounted on the package board 2 before the base board 20 is removed from the redistribution board 4 thus the redistribution board 4 is always reinforced by the base board 20 or the package board 2 therefore the deformation of or the damage to the redistribution board 4 due to the redistribution board 4 being by itself without any reinforcement can be prevented when the base board 20 is removed in step s 31 the first electric conductive layer 22 is exposed from a surface of the redistribution board 4 then in step s 32 the semiconductor elements 6 and 8 are mounted on the redistribution board 4 by flip chip.