Metal nanoparticles and imaging agent containing same

Provided are an MRI imaging agent and a CT imaging agent capable of further increasing the relaxivity of water molecules and having low toxicity to the human body; also provided are metal nanoparticles used as the material of these imaging agents. These metal nanoparticles are formed by coating noble metal ions such as silver ions, gold ions or platinum ions with a manganese porphyrin complex represented in chemical formula (I). Further, this MRI imaging agent and CT imaging agent contain the aforementioned metal nanoparticles.