It is intended to provide a low molecular weight adjuvant which can be administered safely without inducing skin irritation or the like by transdermal or transmucosal administration and is for enhancing immunogenicity of an antigen efficiently. It can be achieved by an immunostimulant adjuvant containing at least one substance selected from the group consisting of aliphatic alcohols, free fatty acids and fatty acid derivatives but does not contain a substance represented by the following formula. (In the formula, R 3 and R 4 may be combined to form a cyclic ring, and R 1 and R 2 independently represent an alkyl side chain having 1 to 16 carbon atoms.)