Provided is a monodisperse agglomerate of a substance-containing vesicle filled with a substance at a concentration higher than conventionally possible. A mixed solution, in which a target substance is included in an aqueous medium, is mixed with a monodisperse agglomerate of a crosslinked vesicle comprising a prescribed polymer which includes a first polymer, i.e. a block copolymer having uncharged hydrophilic segments and first charged segments, and a second polymer having second charged segments carrying a charge opposite to that of the first charged segments, and in which the first polymer and/or the second polymer are/is crosslinked. As a result, the crosslinked vesicle is made to contain the target substance.