Briefly, the present invention provides an apparatus and a method for performing a nuclear magnetic resonance (NMR) measurement on a sample solution containing small molecules and large molecules such as proteins, and provides an apparatus and a method for repeatedly performing the NMR measurement while stably controlling concentration conditions in a sample and changing a concentration of small molecules. A sample solution containing small molecules and large molecules is circularly transferred between a vessel equipped with a nuclear magnetic resonance probe and a control section in which injection and filtration of small molecules are performed. By transferring the sample solution between them, the NMR measurement can be performed while maintaining a concentration of large molecules to be constant and increasing or reducing a concentration of small molecules so that a concentration ratio of the small molecules relative to the large molecules is changed, the two types of molecules being present in the sample solution.