A manufacturing method of a multilayer board includes: forming a metal core layer including a main body, an island portion, and four connection portions, the island portion having a substantially rectangle shape and being located in an opening formed in the main body, the opening having a substantially rectangle shape, the four connection portions connecting side surfaces of four corners of the island portion or side surfaces of vicinities of the four corners of the island portion to a side surface of the main body; forming a first insulating layer on the metal core layer and in the opening; and forming, in the first insulating layer, a hole reaching each of the four connection portions and removing at least a part of each of the four connection portions through the hole to electrically separate the main body and the island portion from each other