This relates to a contrast medium containing a complex of a gadolinium (Gd) type contrast agent and a polymer. More particularly, the contrast medium wherein the polymer capable of phase transit in response to environmental changes, particularly, pH, light or temperature, to develop a different water solubility, and a method for imaging by the use of this contrast medium. By binding a polymer responsive to microenvironmental changes with a Gd type MRI contrast agent, on-off switching of MRI imaging capability is enabled, wherein imaging occurs only on the target sites. Consequently, a highly specific MRI contrast medium can be obtained, whereby a highly specific MRI diagnosis is made possible, wherein only the target sites such as a tumor and a particular site are imaged and otherwise at the site where imaging is not necessary.