To provide a through-hole drilling method and through-hole drilling device using wiring accessories mounted unit that enable a through hole along the inside circumferential shape of an opening of a wiring accessories mounted unit to be drilled easily and precisely at a wall material and that can eliminate a trouble that may arise from the misalignment of the opening to the through hole.

The through-hole drilling device 10 consists of a cutting tool 10a and a hole-drilling bit 14. The hole-drilling bit 14 is provided with a bit shaft 15, and on its end side, a cutting blade part 16 is provided. On the outside circumference of the end of the cutting blade part 16, a non-cutting part 17 is mounted. At the opening side of the inside surface of the surrounding wall of a wiring box 11, guide parts 25 that guide the non-cutting part 17 and the cutting blade part 16 along the inside circumferential shape of the opening of a wiring box 11 in contact with the non-cutting part 17 are provided over the entire circumferential of the surrounding wall.