A method of producing sporophytes of a plant belonging to the order Laminariales wherein gametophytes or young sporophytes of the plant belonging to the order Laminariales can be used as seedlings. A seedling line coated with a polymer gel or a granular material having the polymer gel on the surface thereof is employed as a seedling base material. As the polymer gel, it is preferable to employ one which allows the adhesion of gametophytes and young sporophytes of the plant belonging to the order Laminariales and on which the gametophytes and sporophytes of the plant belonging to the order Laminariales can grow.It is also preferable that the polymer gel contains nutritional salt(s) capable of promoting the growth and maturation of the gametophytes and sporophytes of the plant belonging to the order Laminariales.
