Intended is to provide an MRI contrast agent that tends not to be phagocytized by reticuloendothelial cells and that has improved dispersion stability. The present invention is an MRI contrast agent containing composite particles, wherein the composite particles are those with polymer graft chains bound to surfaces of microparticles at a very high density sufficient to allow steric repulsion to occur between the graft chains, the micoparticles are inorganic microparticles that exhibit superparamagnetism, and the polymer graft chains have a number average molecular weight (Mn) of at least 30,000. Preferably, the MRI contrast agent is for tumor diagnosis.
