A header for a semiconductor package, includes an eyelet having an upper surface, and a lower surface on an opposite side from the upper surface, a metal block having a side surface, and configured to protrude from the upper surface of the eyelet, a lead sealed in a through hole which penetrates the eyelet from the upper surface to the lower surface of the eyelet, and a substrate having a front surface formed with a signal pattern electrically connected to the lead, and a back surface on an opposite side from the front surface. The back surface of the substrate is fixed to the side surface of the metal block. A portion of the back surface of the substrate is exposed from the metal block, and this portion of the substrate is formed with a ground pattern.