To obtain the subject new antiviral agent, comprising a hot water extract of an alga of the genus Spirulina and useful for preventing diseases of fishes due to pathogenic viruses.

This antiviral agent against pathogenic viruses of fishes comprises a hot water extract of an alga of the genus Spirulina. For example, a raw or a dried algal body of Spirulina platensis is used as a raw material. The extraction is preferably carried out by previously suspending an algal body of the genus Spirulina in distilled water in a container, then attaching a cooler to the container, heating the suspension while preventing water loss by evaporation and then keeping the suspension at 80-95°C for 40-60min. Furthermore, the hot water extract assumes a yellow color having the maximum absorption at 480nm when successively adding a 10% phenol and concentrated sulfuric acid to a solution of the extract, produces rhamnose, glucose, fructose, ribose, galactose, xylose, mannose, glucuronic acid and galacturonic acid at the time of carrying out the thermal hydrolysis thereof with trifluoroacetic acid and has preferably 250000-300000 molecular weight (measured by the gel filtration method).