A sample loading plate that includes at least one sample mounting spot that mount a sample thereon is provided with a substrate having a conductive surface and an insulating film that is laminated on the conductive surface of the substrate and that has at least an insulating surface, the insulating film being sparsely formed so that the conductive surface of the substrate is partially exposed at least in the sample mounting spot. Thus, a voltage applied to the sample loading plate can effectively place the sample in an electric field. As a result of which, in a mass spectrometric analysis of the sample, there is no charge up of the sample and appropriate ionization becomes possible.