Wastewater containing arsenic (As) from industrial processes such as non-ferrous smelting has a significant environmental impact due to its high toxicity. There is a need for a method to immobilize arsenic in a stable form suitable for final disposal or long-term storage. Immobilization of arsenic in the form of scorodite (FeAsO4·2H2O), which has excellent chemical stability, is attracting attention as a method for treating wastewater containing high concentrations of arsenic. Scorodite, with its low solubility and high arsenic content per volume, is expected to be an environmentally friendly arsenic fixation method. Among various types of methods, scorodite synthesis using solid iron oxides such as Fe2O3, Fe3O4, and FeOOH as Fe sources has been investigated in recent years with great enthusiasm as a low-temperature scorodite synthesis method under atmospheric pressure. For the synthesis of crystalline scorodite at atmospheric pressure, the in situ oxidation of Fe(II) to Fe(III) in As(V)-containing solution is known to be important. However, the study of the redox reaction of Fe(II) in arsenic-containing solution is very limited. In this presentation, we will report the result of the analysis of the electrochemical behavior in the As(V)-Fe(II)-H2O system using electrochemical methods. In addition, the electrochemical involvement of Fe(II) in the scorodite synthesis by the iron oxide addition method, which significantly accelerates the reaction, will also be presented. The mechanism of the spontaneous redox reaction of the oxidation of Fe(II) to scorodite and the reduction of solid iron oxide containing Fe(III) was elucidated by the electrochemical analysis using solid iron oxide electrodes.