As a substance used as a contrast agent for a method of nuclear magnetic resonance analysis or a method of magnetic resonance imaging, a substance with high selectivity and high sensitivity was demanded. According to the present invention, when a polymer having, in a side chain thereof, a sequence of a 1H—13C—15N, 1H—15N—13C or 1H—13C—13C bond, that is, a structure labeled with stable isotopes of 13C and 15N, is used, the abundance of such a sequence in one molecule can be increased, and hence, high selectivity and higher sensitivity can be attained when used as a contrast agent.