It is important to understand and control corrosion in very small parts. For this purpose, in situ observation of corrosion reactions is necessary. Magnetic fields offer a new way of influencing corrosion reactions. We have investigated the dissolution of a Cu anode in a magnetic field using a holographic laser interferometric microscope. The microscope enables us to quantify the growth of the diffusion layer and the variation in Cu2+ concentration in the vicinity of the electrode. By sandwiching the anode between two doughnut-shaped neodymium magnets, the increase in the surface concentration caused by dissolution is remarkably suppressed, whereas the thickness of the diffusion layer is barely affected. The present results support the hypothesis that mass transport in enhanced in the magnetic field.