An ionizer of the present invention includes a housing, an electron discharge element arranged in the housing, a controller, and a gas introduction, wherein the electron discharge element has a bottom electrode, a surface electrode, and an intermediate layer arranged between the bottom electrode and the surface electrode, and the controller is so set as to apply a voltage to across the bottom electrode and the surface electrode, and so set as to execute a forming process when an electron discharge performance of the electron discharge element is decreased, and the forming process is a process of applying, in a state where a forming process gas is introduced into the housing by using the gas introduction, a forming voltage to across the bottom electrode and the surface electrode using the controller.