The invention provides a shaft used in the medical treatment and a medical instrument, the operability of a medical instrument having a shaft part is greatly improved, and a shaft used in the medical treatment which can reliably fix a core wire is achieved in a simple structure, thereby reducing the manufacturing cost. A tubular base end side shaft (11) to be used and fixed with a linear core wire (16) has a front end part (21) having a spiral slit (23) from a front end edge (22) to a base end side, a cut (25) formed by removing one part of the front end edge (22) while avoiding the slit (23), and a fixed area (26) where the core wire (16) is fixed on an inner wall opposite to the cut (25) when observing the tubular base end side shaft (11) from the front end side.
