The object of the present invention is to provide an assembly substrate which is easily handled and capable of suppressing occurrence of warpage, and offers high productivity and economic efficiency, and its manufacturing method. A work board 100 includes an insulating layer 21 on one surface of a substantially rectangular-shaped substrate 11, and electronic components 41 and a plate-like integrated frame 51 are embedded inside the insulating layer 21. The plate-like integrated frame 51 has a plurality of concave portions 53 arranged in parallel at its inner periphery wall 52a, and arranged on a non-placing area of the electronic components 41 so as to surround a plurality of the electronic components 41 (groups).