An object of the present invention is to develop and provide a method for conveniently introducing a nucleic acid, a peptide, and/or a low-molecular-weight compound into an empty capsid with viral early infection activities kept. The present invention provides a method for producing a drug delivery particle, comprising the steps of: mixing an empty capsid or an empty particle with a drug including a nucleic acid, a peptide, and/or a low-molecular-weight compound in a solution comprising 0.1 to 20% of a surfactant; and keeping the obtained mixed solution at −5 to 50° C. to introduce the drug into the empty capsid or the empty particle.