A deep-hole drilling apparatus capable of preventing a tool mark from growing is provided. The deep-hole drilling apparatus provided for this purpose comprises: a tool (12) that is rotationally driven to drill a deep hole (Wa) in a work (W); an acceleration meter (19) that detects vibration of the tool (12) as the acceleration during deep hole processing; a monitoring apparatus (20) which performs frequency analysis on the acceleration detected by the acceleration meter (19) and which determines that a tool mark is produced in the deep hole (Wa) when the overall value of the whirling frequency and the bending natural frequency within a frequency range not including the torsional natural frequency on torsional vibration, constantly produced during the deep hole processing, exceeds a predetermined threshold (α) ; and an NC apparatus (18) that controls both revolution and feeding speed of the tool (12) on the basis of the determination made by the monitoring apparatus (20).