The influence of ion-acoustic instability on the penetration of a high-frequency field into a plasma is investigated. The slowing-down of the plasma electrons as the instability develops is taken into account by means of a non-linear Ohm's law jz = j0 [1-exp(-αE)] which enables one to consider continuous variations of the depth to which the field penetrates the plasma as a function of the parameter α = E0/Ek, where E0 is the peak electric field value and Ek the field value at which ion-acoustic instability occurs in the plasma. The depth of penetration by the field into the plasma is determined as a function of α. For large α the penetration depth increases by a factor α½ relative to the skin layer thickness of the linear theory (where α ≪ 1).