Insertion device having universal cord with extending transmission member

An insertion device comprising:

• an insertion section configured for insertion into a body cavity;
• an operation section arranged on a proximal side of the insertion section;
• a universal cord extending in a long axis direction, the universal cord having a distal side connected to the operation section, the universal cord including a helical tube, a mesh tube, and an envelope arranged in order radially outward from a central axis;
• a drive source disposed on a proximal side of the universal cord;
• a driven member disposed on the distal side of the universal cord;
• and
• a transmission member provided inside the helical tube of the universal cord in a manner extending along the long axis direction of the universal cord, the transmission member being driven to perform rotation around an extension axis by a driving force of the drive source to transmit the rotation to the driven member, wherein
• a thickness of the helical tube, a diameter of a wire of the mesh tube, a thickness of the envelope, and a hardness of a resin of the envelope are set such that a first torsional resistance of the universal cord around the central axis is higher than a second torsional resistance, around the extension axis, of the transmission member extending from an output portion of the drive source to an input portion of the driven member.