PROBLEM TO BE SOLVED: To provide a method for therapeutically controlling Algae after generation without limiting to the period before generation and at the initial stage of the generation.SOLUTION: Provided is a method for prophylactically or therapeutically controlling Algae by combining an organic acid base sterilant and a dithiocarbamate agent and applying the same to the part to be generated with Algae or the part at which Algae is generated, and also provided is a control agent therefor.