To provide a method for forming a siloxane thin film having excellent adhesion without pinholes, which can control the thickness to be thin and can efficiently form the siloxane thin film by an exceedingly simple method at a low cost.

In a hexadecane solution of octadecyl triethoxy silane having a density of 2wt.%, a cleaned slide glass (borosilicate glass) base plate is immersed and heated at 110°C for ten minutes. Next, the base plate 1 is immersed in a container having a chloroform therein, and the chloroform is stirred for ten minutes for performing chloroform cleaning. Further, it is cleaned by flowing water for 10 minutes, so that a siloxane monomolecular film having a contact angle 97° for water and a thickness of 2nm is formed on the base plate 1.