A sample mounting plate which is used for mass spectrometry according to MALDI process and which is provided with at least one sample loading spot for mounting a sample on a substrate, wherein: a hydrophilic surface produced by a first hydrophilic film is formed within the sample loading spot on a surface of the substrate provided with the sample loading spot; a hydrophobic surface produced by a hydrophobic film is formed on the outside of the hydrophilic surface; and a boundary part, in which a hydrophilic member or a second hydrophilic film having a higher hydrophilicity than the first hydrophilic film is exposed, is formed at the boundary between the hydrophilic surface and the hydrophobic surface.