A method of formation of a capacitor forming part of an electric circuit when producing a circuit board, consisting of forming a valve metal bottom electrode layer and a valve metal oxide dielectric layer on the same, then integrally forming a solid electrolyte layer comprised of an organic semiconductor and a top electrode layer comprised of metal on the same, this integral formation step consisting of the step of holding one surface of metal foil for the top electrode at a bonding wedge and making the other surface of the metal foil carry a powder of the organic semiconductor by compression bonding and heating and the step of compression bonding the organic semiconductor powder carried by compression bonding at the dielectric layer by a bonding wedge through metal foil, whereby a solid electrolyte layer comprised of an organic semiconductor sandwiched between the metal foil and dielectric layer and closely bonded with the two is formed, a capacitor built into a circuit board, a circuit board including a capacitor, and a method of production of the circuit board.