To provide a drilling device capable of accurately processing a small-diameter and deep hole at a high speed by restraining bending of a long drill.

A rotatable swing-resistant mechanism 12 is provided for supporting a tip-leaning side of the rotating rotary drilling tool 11 when drilling the hole on a workpiece W by inserting a tip end of a rotary drilling tool 11. The swing-resistant mechanism 12 has a sleeve member 16 in which the rotary drilling tool 11 is insertably arranged on a mounting member 14 fixed on a device body 10a via a rotary bearing 15. The sleeve member 16 arranged to be exchangeable is arranged so as to coincide with a center axis passing through the workpiece W. The internal diameter of the sleeve member 16 is set up to be a large size so as not to come into contact with the rotary drilling tool 11 in comparison with the external diameter of the rotary drilling tool 11.