It is intended to provide an aqueous solution of a thermoresponsive polymer, which shows an upper critical solution temperature (UCST) and is capable of maintaining a dissolved state, even if it is stored for a long period of time at a temperature at which a ligand such as an antibody or an antigen is not inactivated. A UCST lowering agent is incorporated in a predetermined amount in an aqueous solution of a thermoresponsive polymer which shows a UCST in a state of containing water, thereby lowering the UCST of the aqueous solution to less than the storage temperature, and the resulting aqueous solution is stored.