This paper reports concentration measurement of protein solution under a strong magnetic field. The effect of the magnetic field on the time evolution of the concentration during the growth process of tetragonal crystal of lysozyme was investigated by interferometry. A Mach-Zehnder interferometer was applied to a superconducting magnet for the first time to in situ study the concentration profile beyond the solid/liquid interface during the crystal growth. Results showed that during the growth process, decrease in the lysozyme concentration under homogeneous magnetic field was slowed down as compared with those without magnetic field. It was postulated that solutal convection during the crystal growth might have been influenced by the magnetic field. However, study of the convection under different magnetic fields indicated little effect of the magnetic field on the convection. Furthermore, after checking the solubility under magnetic field, we found that there was little effect of magnetic field on the solubility. Further investigation with respect to the growth and dissolution processes and mechanisms under magnetic field are in progress.