To provide an optical level for machine tool, which facilitates vertical drilling of a hole on a floor surface.

An optical level 10 is mounted on a machine tool 8 having a body portion 82 for holding a drill 81 coaxially. The optical level 10 includes light sources 11 to 14 for emitting diffused light beams d1 to d4, and a holding means 6 for holding the light sources 11 to 14 to the body portion 82 so that the optical axes of the diffused light beams d1 to d4 may be emitted in parallel with the drill 81 to irradiate a surface FL to be machined. The holding means 6 includes the light sources 11 to 14 arranged so that the light-emitting ends of the diffused light beams d1 to d4 have the same distance away from the tip of the drill 81 and the light-emitting ends of the diffused light beams d1 to d4 are positioned at the virtual cross point of the circumference centering on the tip of the drill 81 and the straight line passing the tip of the drill 81. The posture of the machine tool 8 is corrected to form the contour of the diffused light beams d1 to d4 irradiating the surface FL to be machined from an elliptic shape to a circular shape to drill a vertical hole on the surface FL to be machined.