In order to propose and verify a method of non-destructive inspection for cracks in existing steel structures by FSM supported with an electric field analysis, a series of experiments and analyses were carried out. It was confirmed that the electric field pattern of the actual-sized steel plate deck in which cracks did not existed could be simulated with high accuracy by the electric field analysis. The electric field pattern was largely changed when a crack was initiated and propagated in the deck. By comparing the electric field pattern actually obtained by FSM with that obtained by the analysis simulating the virgin situation, whether cracks exist or not could be judged and evaluated if there was a difference between the both electric field patterns.