The steady-state reduction current of the hydrogen ion in a 1:1 strong acid without adding supporting electrolyte at a microelectrode is known to be independent of the diffusion coefficient of the anion, although the anion diffuses to the electrode together with the hydrogen ion in order to maintain electroneutrality. This paper aims at resolving this inconsistency of the diffusion of the anion by investigating transient reduction currents of the hydrogen ion without supporting electrolyte. The time-dependent diffusion equation associated with migration was solved under the condition of the potential step toward the limiting current-domain at a hemi-spherical electrode. The theoretical transient current has a linear relation with the inverse square root of the time. The anion behaves as if it were electroactive. The slope of the line is expressed by an average of diffusion coefficients of the hydrogen ion and the interacted anion. Chronoamperometric measurements were made in hydrochloric acid including various concentrations of supporting electrolyte.