PROBLEM TO BE SOLVED: To provide a method for promoting the growth of Laminaria that can promote the growth of Laminaria more effectively, in a method that supplies slag-mixed fertilizer material in real sea area.SOLUTION: A method for promoting the growth of Laminaria comprises: a first fertilization step of installing a first fertilization body, in which a water permeable bag is filled with a mixture of a humic acid-containing material and steel slag, in a sea area in a gametophyte maturation period in a life history of Laminaria; and a second fertilization step of additionally installing a second fertilization body, in which the water permeable bag is filled with the humic acid-containing material, in at least either the growth-period or the extension period in the life history of Laminaria.COPYRIGHT: (C)2015,JPO&INPIT