An endoscope system comprising: a side-view endoscope including an elongated insertion portion provided with a bendable bending portion and a flexible tube portion having flexibility and provided on a proximal end side of the bending portion, the insertion portion being insertable into a lumen; balloons (10a-10d) which are expandable and contractable in a diameter direction of the insertion portion in accordance with air feeding and air sucking by an air feeding/sucking device and which are disposed on a distal end portion of the flexible tube portion, the balloons (10a-10d) being configured to move a one-side surface, which is located nearer to a distal end side than the bending portion of the insertion portion, in parallel in the diameter direction of the insertion portion, separately from bending of the bending portion, the one-side surface being located in a circumferential direction along an insertion direction of the insertion portion and being parallel to a central axis of the insertion portion; an expansion/contraction mechanism (11a-11d, 70c-70e) provided in a linked manner to the balloons (10a-10d) and configured to expand and contract the balloons (10a-10d), and an objective lens disposed on the one-side surface, characterized in that four balloons (10a-10d) are disposed in the circumferential direction of the distal end portion of the flexible tube portion such that the balloons (10a-10d) are opposed to each other at line-symmetric positions with respect to either one of a first axis which passes the central axis to be parallel to a field of view direction of the objective lens provided on the one-side surface or a second axis which passes the central axis and is orthogonal to the first axis, and the expansion/contraction mechanism (11a-11d, 70c-70e) includes a conduit (11a-11d) connected to the respective balloons (10a-10d) and a syringe to which the conduit (11a-11d) is connected and which includes inside thereof a piston and a valve for changing over whether or not to introduce air from the air feeding/sucking device into the inside of the syringe, wherein the endoscope system is configured such that, in a state where the field of view direction of the objective lens and an insertion direction of a treatment instrument protruded from an aperture formed on the one-side surface coincide with each other by fixing a bending angle of the bending portion, when the air is supplied from the air feeding/sucking device to one of the opposed balloons (10a-10d) and another one of the opposed balloons (10a-10d) via the conduit (11a-11d), the opposed balloons (10a-10d) expand in the diameter direction, and when the piston moves inside the syringe in a state where the valve is closed and thereby the air inside the one of the opposed balloons (10a-10d) is moved to the another one of the opposed balloons (10a-10d), the one of the opposed balloons (10a-10d) contracts in the diameter direction and the another one of the opposed balloons (10a-10d) expands in the diameter direction interlockingly with the contraction, thereby moving the one-side surface in parallel to a position close to a papilla in the field of view direction by utilizing the expansion of the another one of the opposed balloons (10a-10d), while making the field of view direction and the insertion direction coincide with each other. a side-view endoscope including an elongated insertion portion provided with a bendable bending portion and a flexible tube portion having flexibility and provided on a proximal end side of the bending portion, the insertion portion being insertable into a lumen; balloons (10a-10d) which are expandable and contractable in a diameter direction of the insertion portion in accordance with air feeding and air sucking by an air feeding/sucking device and which are disposed on a distal end portion of the flexible tube portion, the balloons (10a-10d) being configured to move a one-side surface, which is located nearer to a distal end side than the bending portion of the insertion portion, in parallel in the diameter direction of the insertion portion, separately from bending of the bending portion, the one-side surface being located in a circumferential direction along an insertion direction of the insertion portion and being parallel to a central axis of the insertion portion; an expansion/contraction mechanism (11a-11d, 70c-70e) provided in a linked manner to the balloons (10a-10d) and configured to expand and contract the balloons (10a-10d), and an objective lens disposed on the one-side surface,