A capacitor built-in interposer of the present invention, includes a base resin layer, a capacitor first electrode provided to pass through the base resin layer and having projection portions projected from both surface sides of the base resin layer respectively whereby the projection portion on one surface side of the base resin layer serves as a connection portion, a capacitor dielectric layer for covering the projection portion of the first electrode on other surface side of the base resin layer, and a capacitor second electrode for covering the dielectric layer, wherein a plurality of capacitors each constructed by the first electrode, the dielectric layer, and the second electrode are arranged and aligned in a lateral direction in a state that the capacitors are passed through the base resin layer.