A vesicle including metal microparticles, the vesicle configured having a membrane formed by a first polymer of (a) or (b) and a second polymer of (c) or (d) (the combination of (b) and (d) being excluded), a portion of a cationic segment and an anionic segment in the polymers being cross-linked in the membrane. First polymer: (a) a block copolymer (I) having a non-charged hydrophilic segment and a cationic segment, (b) an amino acid polymer (I) having a cationic segment Second polymer: (c) a block copolymer (II) having a non-charged hydrophilic segment and an anionic segment, (d) an amino acid polymer (II) having an anionic segment.
