This study attempts to apply low frequency eddy current testing to measure the thickness of double-layered plates which consist of two 304 austenitic stainless steel plates with a thickness of about 30 mm and an air gap with a thickness of 80 mm. The results of numerical simulations show that a change in the magnetic flux density due to the thickness change of the plates appears at the center of the coil when the diameter of the exciting coil is as large as 300 mm and the exciting frequency is in the low tens of Hertz. Experimental verifications were conducted using a magneto-impedance sensor whose sensitivity was 1 mV/nT and sensing range was from 0.2 nT to 2000 nT. We used a large diameter pancake exciting coil and a compensating coil to make up for the short of the range of the magnetic sensor during the measurements. The signals of the sensor situated at the center of the exciting coil changed with the thickness of the double-layered plates, which agrees with the result of the simulations.