There is disclosed a nuclear magnetic resonance imaging agent comprising a complex compound composed of (a) dialdehyde-saccharide having a molecular weight of from 500 to 10,000, at least one of constituent monosaccharides of which is oxidation-cleaved, (b) at least one complexing agent which is chemically coupled to an aldehyde group of the dialdehyde-saccharide and (c) a paramagnetic metal ion which is chemically coupled to the complexing agent.
