This paper proposes a new nondestructive evaluation method for detecting cracks, voids, and other hidden defects inside concrete structures, called "frequency differential electrical impedance scanning (fdEIS)." The primary benefit of fdEIS over the conventional nondestructive methods is that it is possible to determine the thickness of the voids. In fdEIS, we inject a sequence of electrical currents with various frequencies through the tested concrete wall by applying a sinusoidal voltage difference between a surface electrode and a scan probe, which are placed on opposite surfaces of the wall. Through the probe, we measure the derivative d/dw gw of exit currents (Neumann data) with respect to the angular frequency variable w. We find the fundamental concept in fdEIS relating the thickness of the voids to d/dw gw and derive an approximation formula for estimating the thickness of the voids. We demonstrate the performance of our method in numerical simulations.