After a sample such as a biomedical tissue section is attached to an electrically-conductive slide glass (S1), the film layer of a matrix substance is appropriately formed by vapor deposition so as to cover the sample (S2). The crystal of the matrix substance in the film layer is very fine and uniform. Subsequently, the slide glass on which the matrix film layer is formed is placed in a vaporized solvent atmosphere, and the solvent infiltrates into the matrix film layer (S3). When the solvent sufficiently infiltrated is vaporized, a substance to be measured in the sample takes in the matrix and re-crystallized. Furthermore, the matrix film layer is formed again on the surface by the vapor deposition (S4). The added matrix film layer absorbs excessive energy of a laser beam during MALDI, which suppresses the denaturation of the substance to be measured and the like, so that high detection sensitivity can be achieved while high spatial resolution is maintained.