Global warming is one of the most serious problems facing mankind in the 21 st Century. Sea ice plays an important role in climate variability. Especially, sea ice thickness is one of the key parameters for understanding the heat flux of thin sea ice area. In this study, in situ measurements of thin ice thickness were performed simultaneously with FORMOSAT-2/RSI observation in Saroma Lake and the off the coast of Abashiri Bay, Hokkaido, Japan. The comparison of the in situ measurement of thin ice thickness and FORMOSAT-2/RSI data showed linear relationship between the two. But, on the other hand, ice thicker than 20cm did not show clear relationship of the two. In this study, the authors will discuss about the possibility and limitation of using FORMOSAT-2/RSI data for thin ice thickness monitoring.