An object of the present invention to provide: a nanoparticle, which can be produced without the use of surfactants and synthetic polymers, which has a controllable size and is stable in the acidic range, and which further contains an active substance and a method for producing the same. The present invention provides a casein nanoparticle which contains an active substance and has an average particle size between 10 nm or more and less than 300 nm, which is produced by the following steps (a) to (c):(a) a step of mixing casein into a basic aqueous medium between pH 8 or more and less than pH 11(b) a step of adding at least one type of active substance to the solution obtained in the step (a) and(c) a step of injecting the solution obtained in the step (b) into an acidic aqueous medium at pH 3.5 to pH 7.5.
