PURPOSE: To easily obtain an inverted trapezoid-shaped groove whose section the groove bottom of is parallel with the surface of a material to be cut, by grinding a diamond cutting tool and moving this cutting tool onto the thin film to be cut in the fixed state and forming the groove with the diamond cutting tool.

CONSTITUTION: A thin film 11 to be cut is formed in the area on a glass disk surface 10, which is ground to have parallel planes, by Cu sputtering or the like, and an active thin film 12 is formed on the area on the glass disk surface 10 by sputtering of materials like Ni, chemically active to diamond. A diamond cutting tool 14 is fixed to a cutting tool holder 15, and a certain vertical load is allowed to act, and the diamond cutting tool 14 has the edge 16 ground while being pressed onto the active thin film 12. An air spindle 13 is rotated and the diamond cutting tool 14 is moved in the radial direction of the glass disc 10 while having the edge 16 pressed onto the thin film 11 to be cut, thereby spirally forming the groove. Thus, the inverted trapezoid-shaped groove whose section a groove bottom 18 of is parallel with a surface 17 of the thin film 11 to be cut is formed.