Endoscope

1. An endoscope comprising:

an insertion portion provided with a bending portion at a distal end part;

an operation portion coupled to the insertion portion;

four operation wires insertedly arranged inside the insertion portion and the operation portion, the four operation wires causing the bending portion to bend by being pulled/slackened;

one operation member provided on the operation portion, the operation member having a rotation portion and being capable of performing a bending operation of the bending portion in four directions by being inclined with the rotation portion as a center;

a rotation member provided in the operation portion and having a proximal end side and a distal end side, the proximal end side being connected to an opposite side of the operation member with the rotation portion as a center, and the rotation member pulling/slackening the four operation wires by being inclined with the rotation portion as a center in conjunction with an inclination operation of the operation member; and

an amount-of-operation-force reducing portion, an end portion of which is movably held on the distal end side of the rotation member, the amount-of-operation-force reducing portion applying rotation torque to the rotation member in a direction in which the rotation member is inclined, in response to inclination of the operation member, to reduce an amount of operation force of the operation member, wherein

the rotation torque of the amount-of-operation-force reducing portion increases with an increasing displacement of the operation member to offset and reduce the amount of operation force required for the inclination operation of the operation member.

2. The endoscope according to claim 1 , wherein, as for the rotation torque, a small rotation torque is applied to the rotation member when an amount of displacement of the operation member operated is small, and a large rotation torque is applied to the rotation member when the amount of displacement is large, to offset and reduce the amount of operation force required for the operation member.

3. The endoscope according to claim 1 , wherein the amount-of-operation-force reducing portion is provided with an elastic member for applying the rotation torque to the rotation member in a rotation direction of the rotation member.

4. The endoscope according to claim 3 , wherein the elastic member is a compression spring urging the rotation member to apply the rotation torque applied to the rotation member in the rotation direction of the rotation member.

5. The endoscope according to claim 4 , wherein the compression spring is arranged so as to urge force toward a rotation center of the rotation member so that the rotation torque is not applied to the rotation member when the bending portion is in a neutral-position state in which the bending portion is straight without being bent.

6. The endoscope according to claim 1 , wherein the plurality of operation wires are provided with pulling members providing a state of being tensioned at least when the bending portion is in the neutral-position state in which the bending portion is straight without being bent, even in a state of being slackened.

7. The endoscope according to claim 6 , wherein the pulling members pull the plurality of operation wires in a direction having a predetermined angle relative to a longitudinal direction of the plurality of operation wires.

8. The endoscope according to claim 6 , wherein the pulling members are slidably connected to the plurality of operation wires.

9. The endoscope according to claim 6 , wherein the pulling members are fixedly connected to the plurality of wires.

10. The endoscope according to claim 6 , wherein the pulling members are pulling springs pulling the plurality of operation wires.