A groove having a depth greater than 1 μm is efficiently cut out on glass as a work by use of a super-hard ball end mill. A groove having a depth equal to or above 1 μm is formed on a work made of a glassy inorganic material or a hard, brittle material by one session of cutting operation without generating damage while rotating a ball end mill at a high speed. A rotating shaft of the ball end mill has a relative tilt angle in a feeding direction of the ball end mill with respect to a cutting surface of the work when subjecting the work to cutting while rotating the ball end mill at a high speed. In this way, a cutting groove having a width of 200 μm and a depth of 20 μm is formed on a glass substrate as the work by one session of cutting operation.