To provide a method for examining seaweedproofing efficacy of a coating film, capable of accurately and surely judging seaweedproofing efficacy of the coating film formed by a coating material in which a seaweedproofing agent is mixed.

A seaweedproofing agent having a prescribed concentration is mixed into a coating material and the coating material is applied to a surface which comes into contact with a culture medium of a culture vessel 21 to form a coating film 31. Then, a culture solution in which algal cell is contained is housed as a sample 32 and an algal cell in the sample 32 of the culture vessel 21 is cultured in a culture room. Absorbance of the sample 32 in the culture vessel 21 in the culture room is calculated based on measuring result by a permeation type chlorophyll measuring apparatus 10 and sequential change in cell density of algal cell of the sample 32 is obtained according to the absorbance and seaweedproofing effect of coating film in which seaweedproofing agent is mixed is judged based on sequential change in cell density of the obtained algal cell.