An endoscope system comprising: an endoscope including: an insertion section to be inserted into a body; and an operation section connected to a proximal end side of the insertion section, the insertion section having a distal end hard portion, a bending portion connected to a proximal end side of the distal end hard portion and a flexible portion connected to a proximal end side of the bending portion; wherein the flexible portion includes: a low flexural rigidity portion that is positioned on a distal end side of the flexible portion; and a flexural rigidity varying portion positioned at the proximal end side of the low flexure rigidity portion and having a flexural rigidity which increases from a low flexural rigidity portion side toward a proximal end side, the flexural rigidity varying portion having an average rate of change of flexural rigidity in a longitudinal axial direction of the insertion section, the average rate of change of flexural rigidity being larger than an average rate of change of flexural rigidity in the low flexural rigidity portion in the longitudinal axial direction of the insertion section, characterized in that the endoscope system includes an insertion auxiliary tool including a tube body having a distal end opening, a proximal end opening and an insertion passage into which the insertion section is inserted from the proximal end opening, the insertion section being movable back and forth along a center axis direction of the tube body, the tube body being configured to have a length that allows at least a part of the flexible portion to project from the distal end opening when the insertion section is positioned at a distal end position within a back-and-forth movable range in which the insertion section is movable with respect to the tube body, and when the insertion section is positioned at a distal end position within the back-and forth movable range with respect to the tube body, a proximal end position of the low flexural rigidity portion is positioned on a proximal end side with respect to the distal end opening of the tube body, in the longitudinal axial direction of the insertion section. an endoscope including: an insertion section to be inserted into a body; and an operation section connected to a proximal end side of the insertion section, the insertion section having a distal end hard portion, a bending portion connected to a proximal end side of the distal end hard portion and a flexible portion connected to a proximal end side of the bending portion; wherein the flexible portion includes: a low flexural rigidity portion that is positioned on a distal end side of the flexible portion; and a flexural rigidity varying portion positioned at the proximal end side of the low flexure rigidity portion and having a flexural rigidity which increases from a low flexural rigidity portion side toward a proximal end side, the flexural rigidity varying portion having an average rate of change of flexural rigidity in a longitudinal axial direction of the insertion section, the average rate of change of flexural rigidity being larger than an average rate of change of flexural rigidity in the low flexural rigidity portion in the longitudinal axial direction of the insertion section, a low flexural rigidity portion that is positioned on a distal end side of the flexible portion; and a flexural rigidity varying portion positioned at the proximal end side of the low flexure rigidity portion and having a flexural rigidity which increases from a low flexural rigidity portion side toward a proximal end side, the flexural rigidity varying portion having an average rate of change of flexural rigidity in a longitudinal axial direction of the insertion section, the average rate of change of flexural rigidity being larger than an average rate of change of flexural rigidity in the low flexural rigidity portion in the longitudinal axial direction of the insertion section, characterized in that the endoscope system includes an insertion auxiliary tool including a tube body having a distal end opening, a proximal end opening and an insertion passage into which the insertion section is inserted from the proximal end opening, the insertion section being movable back and forth along a center axis direction of the tube body, the tube body being configured to have a length that allows at least a part of the flexible portion to project from the distal end opening when the insertion section is positioned at a distal end position within a back-and-forth movable range in which the insertion section is movable with respect to the tube body, and when the insertion section is positioned at a distal end position within the back-and forth movable range with respect to the tube body, a proximal end position of the low flexural rigidity portion is positioned on a proximal end side with respect to the distal end opening of the tube body, in the longitudinal axial direction of the insertion section.