It was difficult to be eaten by the reticulum endothelium type cell, at the same time, distributed stability improved, offer of the MRI contrast medium is designated as purpose. As for this invention, being the MRI contrast medium which includes the composite particle, as for the aforementioned composite particle, until three-dimensional rally occurs between the graft chains, it is the composite particle which the high-molecular graft chain joins to the particulate surface at super high density, the aforementioned particulate is the inorganic particulate which shows super paramagnetism, the Kazuhei equal molecular weight of the aforementioned high-molecular graft chain (Mn), is 30,000 or more. As for the aforementioned MRI contrast medium, it is desirable to be for tumor diagnosis.細網内皮系細胞に貪食されにくく、かつ、分散安定性が向上した、MRI造影剤の提供を目的とする。本発明は、複合粒子を含むMRI造影剤であって、前記複合粒子は、グラフト鎖間で立体反発が生じるまでに超高密度で高分子グラフト鎖が微粒子表面に結合した複合粒子であり、前記微粒子は、超常磁性を示す無機微粒子であり、前記高分子グラフト鎖の数平均分子量(Mn)が、30,000以上である。前記MRI造影剤は、腫瘍診断用であるのが好ましい。
