An electrochemical atomic force microscope (EC-AFM) was used to study the reaction of a lead dioxide electrode in sulfuric acid solution, while the reaction corresponding to what occurs at the positive electrode of a lead-acid battery took place. We observed continuous in situ morphology change of the lead dioxide electrode and the potentiostatic transient, when the oxidation and reduction potentials were applied to the electrode. As a result, it was confirmed that lead sulfate crystals began to deposit from the moment when the reduction potential was applied and the reduction transient showed a sharp peak shape at the same time. On the other hand, the dissolution of lead sulfate crystals was delayed from the moment when the oxidation potential was applied. It took about 2min for all the crystals to dissolve, and the oxidation transient was recorded as a broad shaped curve.