This study investigated how inert supporting electrolytes (SEs), which increase the electrical conductivity, affect electrochemical ozone production (EOP) on a boron-doped diamond (BDD) electrode. Regardless of the SE species, the EOP was suppressed about 60% in a SE concentration of 1 mM for which the conductivity is similar to that of tap water compared to deionized water. The production of H₂O₂, which is known to be generated by the combination of OH, was also suppressed. On the other hand, the formation of OH was not significantly affected by the presence of SEs, an intermediate for ozone production. Consequently, suppression of EOP by SE can be explained by physical interference from the diffusion or combination of OH by the SE anions concentrated near the electrode surface. This study contributes by providing a better mechanistic understanding of the effect of SEs on EOP in a solid polymer electrolyte/BDD system.