Provided is a polymeric micelle pharmaceutical preparation that can increase the ratio of contrast at tumor site to background contrast in a short period of time after administration of a lactosome and can suppress the ABC phenomenon so that the lactosome can be administered more than once within a short span. A branched-type amphiphilic block polymer comprising: a multi-branched hydrophilic block comprising sarcosine; and a hydrophobic block comprising polylactic acid. The branched-type amphiphilic block polymer, wherein the number of branches of the hydrophilic block is 3. A molecular assembly comprising the branched-type amphiphilic block polymer. The molecular assembly further comprising a linear type amphiphilic block polymer.