The present invention relates to a sample plate that is preferably constituted by a metal plate on which a membrane is affixed by a conductive double-sided tape. In the case where a sample, developed on a medium such as an electrophoresis gel, is transferred onto the membrane, in the same manner as a conventional blotting operation, a medium such as gel and the sample plate are sandwiched between flat plate electrodes so that the medium and the membrane are made in closely contact with each other, and a voltage is applied between both of the electrodes. Thus, the sample developed on the medium is transferred on the membrane through electrophoresis. Upon laser desorption ionization mass spectrometry, the sample plate is compatibly used as a plate for mass spectrometry.