PROBLEM TO BE SOLVED: To provide a novel therapeutic agent useful for prevention and/or treatment of object diseases containing Fucus vesiculosus rhizoid or extract thereof as an active ingredient by resolving the mechanism of hair restoration and growth effects of the Fucus vesiculosus rhizoid by the extract thereof, while considering a high concentration extraction of an activity fraction of the Fucus vesiculosus rhizoid.SOLUTION: The invention provides 5&alpha-reductase inhibitor containing Fucus vesiculosus rhizoid and extract thereof as an active ingredient.