Used is a sample holder for MALDI mass spectrometry, which has a CuO secondary particle as a laser-beam-absorbing matrix and in which the secondary particle comprises an aggregate of CuO primary particles having an average particle diameter of 100 nm or smaller and has an uneven surface arising from the shape formed by the primary particles constituting the outermost surface of the secondary particle. As the CuO secondary particle, usable is one derived from a CuO powder produced by baking basic copper carbonate in air at 200 to 300° C., and the basic copper carbonate is produced in a process of mixing an aqueous ammonium hydrogencarbonate solution and an aqueous copper nitrate solution. The CuO secondary particle has an average particle diameter of, for example, from 0.3 to 10 μm.