This paper evaluates the applicability of the extremely low frequency eddy current testing to the monitoring of wall thinning appearing on the inner surface of pipes. Magneto-Impedance sensor arrays are used to detect signals due to wall thinning. Plates made of SS400 with artificial grooves are prepared to simulate wall thinning in a large diameter pipe. We carry out numerical simulations and experiments to evaluate the effect of the depth and width of the grooves on signal. The results show the maximum amplitude and the full width at half maximum of signals correspond approximately to the depth and width of the grooves, respectively.