The present invention provides a safe polymer nanoparticle composite with few side effects, and an MRI contrast agent incorporating said polymer nanoparticle composite. The polymer nanoparticle composite is capable of specifically accumulating on a tumor tissue to selectively extract the tissue, exhibiting high contrast even when used in small amounts, and enabling imaging over prolonged periods of time. This polymer nanoparticle composite is characterized by containing a block copolymer that includes a non-charged hydrophilic polymer chain segment and an anionic polymer chain segment, and MnCaP.
