There is a known method for elongating the cutting tool life by using adhesion materials made from work materials as protective film. These adhesion materials are called belag. However, the belag formation mechanism has not yet been clarified. Therefore, the purpose of this study is to investigate the belag formation mechanism by observing the adhesive materials on the cutting edge and the surface of the chip after cutting a medium carbon steel (JIS : S50C) through SEM, TEM, and TOF-SIMS. The results showed the following : First, oxide mostly containing manganese formed on both the rake face and the honing of the cutting tool, but oxide on the rake face also contained aluminum, while that on the honing contained silicon. Second, the iron oxide that is different from the composition of belag was formed on surface of the chip. Third, aluminum, manganese, and silicon were concentrated near the interface of the iron oxide layer and the metal layer on the chip. Finally, the composition of belag was similar to the oxide of S50C formed in the low oxygen potential. These data suggest that belag was formed by the preferential oxidation of soluble aluminum, manganese, and silicon in the work material.