Ag deposition was investigated using separately-excited (passive technique) EQCM in a room-temperature ionic liquid. The mass changes during silver deposition and dissolution were observed with the current efficiencies of nearly 100 % during potential sweep, constant potential step, and constant current step experiments. The product of the viscosity and density of the electrolyte near the electrode, ηρ, can be estimated by the resonance resistance, which can be monitored simultaneously with the resonance frequency. The change in the ηρ value during Ag deposition was consistent with the simulation results of the concentration profile of Ag(I) near the electrode. During outer-sphere electron transfer reaction between Fc+ and Fc, no significant change in the mass and the ηρ value was observed.