To steer a needle around delicate or bony structures during needle insertion for targeting tumor in minimally invasive surgery, we proposed a novel continuum robot mechanism named “Active Sheath” and applied the mechanism to a novel steerable needle with a bevel tip. The mechanism contains only two units, that is, a guide tube and a loop-shaped inner arm inserted into the tube. When the base of each arm is pulled or pushed, the arm could be deflected in the guide and therefore the guide tube could also be deflected following the arm deformation. The control system of the steerable needle insertion was derived as a nonholonomic affine model to calculate the needle paths numerically. The prototype of the needle with 2.1[mm] outer diameter was developed and evaluated for the steering performance to measure the tip location and orientation by inserting it into an agar phantom. The path steered by using inner arm could be deflected maximum 24 times larger than that of the non-driving needle deformed by reaction force only. From the results we concluded that the proposed mechanism has the potential to target tumors safely by avoiding delicate or bony structures during needle insertion.