A close-wound coil having a good rotation transmission performance, and a medical treatment tool using the close-wound coil. The close-wound coil is formed by winding a wire (18) spirally and closely over a predetermined length around a predetermined first axis (O1). The wire (18) is formed, so that in the section (S) vertical to a wire axis (O2), the second moment of area concerning a second axis (O3) passing across the center (O2) of the section (S) and vertical to the first axis (O1) is smaller than the second moment of area concerning a third axis (O4) passing across the center of the section (S) and vertical to the second axis (O3) and parallel to the first axis.
