The present invention relates to: micellar aqueous compositions formed by chitosan derivatives in which carboxy-C 1-5 alkyl groups have been introduced as the hydrophilic groups and alkyl groups of 9 to 21 carbon atoms or aliphatic acyl groups of 9 to 21 carbon atoms have been introduced as the hydrophobic groups, and hydrophobic drugs; and a method of solubilizing hydrophobic drugs by adding the chitosan derivatives and hydrophobic drugs into an aqueous solvent to form micellar aqueous compositions. According to the present invention, drugs that could not be used for injection due to their insolubility in water can be formulated into injections, and also the effect of promoting absorptivity of drugs that were poorly absorbed orally etc. due to its poor solubility can be expected.