A header assembly includes a header holding a plurality of electrode pins to be insulated from one another, an ignition element mounting capacitor having at the center of its outer circumferential surfaces external terminal electrodes for electrically connecting to an ignition element mounted on the capacitor, an IC having first, second, and third electrode pads to be electrically connected to end electrodes of the ignition element mounting capacitor and the external terminal electrodes and further having connection electrodes to be electrically connected to the electrode pins of the header for communication with the external. The IC is located on the header, and the ignition element mounting capacitor is located on the IC and electrically connected to the electrode pins through the connection electrodes provided on the IC. The invention provides a squib enabling a size to be minimal and quality to be improved, while maintaining high productivity, and hence provides the header assembly and the ignition element mounting capacitor as components of the squib, and further a gas generator with the squib installed therein for an air bag, and a gas generator having the squib installed therein for a seat belt pretensioner.