A matrix film forming device including a first electrode plate including a mounting surface on which a sample plate P is mounted; a second electrode plate arranged facing said mounting surface; a nozzle which sprays a liquid containing a matrix substance by an electrospray process into the space between the first electrode plate and the second electrode plate and is arranged such that the first electrode plate and second electrode plate are not present over the central axis of the spray stream; and an electric field forming device which forms, between the first electrode plate and the second electrode plate, an electric field causes liquid drops containing charged matrix substance contained in the spray stream to move toward said mounting surface.