The present invention has an object to achieve soft ionization more easily when a slight amount of substance is ionized under an atmosphere pressure. The present invention provides an ionization method for a substance contained in a liquid, including: supplying the liquid to a substrate from a probe and forming a liquid bridge made of the liquid containing the substance dissolved therein, between the probe and the substrate; oscillating the substrate; and generating an electric field between an electrically conductive portion of the probe in contact with the liquid and an ion extraction electrode.