A vesicle comprising fine metal particles, which has a membrane formed from both a first polymer of (a) or (b) shown below and a second polymer of (c) or (d) shown below (with the proviso that a combination of (b) and (d) is excepted), wherein partial crosslinking occurs between a cationic segment and an anionic segment in the above polymers: First polymer: (a) a block copolymer (I) having both an electrically 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 both an electrically non-charged hydrophilic segment and an anionic segment (d) an amino acid polymer (II) having an anionic segment.
