The present invention principally addresses the problem of providing a novel molecular assembly containing functional RNA and capable of introducing functional RNA such as siRNA and shRNA into a cell by in vivo administration, and also delivering the functional RNA to a diseased site. The present invention provides, for example, a molecular assembly characterized by containing the following as essential components thereof: an amphiphilic block polymer A having a hydrophilic block containing a sarcosine chain and a hydrophobic block containing a lactic acid chain； RNA modified by a compound having a long-chain hydrophobic group such as a polylactic acid chain or a polylactic acid-polysarcosine copolymer chain； a plasma-membrane-binding compound (CPP, etc.)； and a photosensitizer which may bond to one or more of these components. The present invention is useful as an RNA delivery system (DDS) into tumor cytoplasm and as a cancer preventive or therapeutic agent.
