The present invention provides a novel amphiphilic substance, a nanoparticle using the novel amphiphilic substance, which can be used as a nanocarrier having high biocompatibility, a drug delivery system and a probe useful for the system, and, a molecular imaging system and a probe useful for the system. An amphiphilic block polymer comprising a hydrophilic block; and a hydrophobic block, wherein the hydrophilic block is a hydrophilic polypeptide chain having 10 or more sarcosine units, and the hydrophobic block is a hydrophobic molecular chain comprising units selected from the group consisting of amino acid units and hydroxyl acid units as essential structural units, and having 5 or more of the essential structural units.