The remote field eddy current testing (RFECT) is usually used for inspection of ferromagnetic small tubes with bobbin coils. As a new aspect of RFECT, YuShi Sun introduced RFECT for flat plates using pancake coils. However, a principle of RFECT using pancake coils is still unclear, and a few studies on this kind of RFECT have been reported so far. The aim of this study is to evaluate detection and sizing capabilities of RFECT using pancake coils especially for applying this technique to evaluation of local wall thinning in large diameter pipes from the outside. The present paper shows relationships between detection signals and defect shapes obtained by numerical simulations, and also compares one of the numerical results with a corresponding actual experimental result. The results indicate the possibility of estimating a defect shape from detection signals and the possibility of detecting a defect with a relatively large lift-off height or on the bottom side of double plates.