Provided is a metal core board which has a preferable function for a vehicle-mountable junction box, specifically, is reduced in size or improved in mounting efficiency. A metal core board for a vehicle-mountable junction box, which is mountable on the vehicle-mountable junction box, wherein a core plate forming an intermediate layer of the metal core board has a plurality of slits and an island surrounded by separating connection parts present between the slits; and in the state where the island is held between insulating layers stacked on both of two surfaces of the core plate, through-bores 46 are formed at the separating connection parts to remove the separating connection parts and the island is made electrically independent from a remaining part of the core plate while being held between the insulating layers. Thus, a plurality of circuits can be formed.