Endoscope apparatus and endoscope system providing angular orientation between overtube and endoscope insertion section inserted in overtube

An endoscope system, comprising:

• an endoscope insertion portion configured to be inserted into a subject the endoscope insertion portion comprising:
• a distal end portion;
• and
• a second conduit configured to cause a proximal end side and a distal end side of the endoscope insertion portion to communicate with each other, the second conduit being configured for insertion of a treatment instrument into an inside of the subject from a second opening on a distal end face of the distal end portion;
• an overtube configured to cover at least an outer periphery of the distal end portion of the endoscope insertion portion, the overtube comprising a first conduit provided with a first opening, the first conduit being configured to cause an inside of the subject and an outside of the subject to communicate with each other;
• wherein the overtube having one of a groove and a projection and the endoscope insertion portion having an other of the groove and the projection, the groove and the projection being configured to mate together to define an angular orientation of the first opening relative to the distal end portion about a longitudinal axis of the distal end portion when the overtube covers at least the outer periphery of the distal end portion;
• a treatment instrument configured to be inserted into the subject from the outside of the subject via the second conduit;
• a high-frequency current generator configured to cauterize a part of the subject by giving energy to the treatment instrument in the subject;
• and
• a suction pump to which the first conduit connects to discharge mist generated when the high-frequency current generator cauterizes a part of the subject to the outside of the subject from the inside of the subject via the first conduit,
• wherein the endoscope insertion portion includes an observation optical system configured to acquire an optical image of the inside of the subject, and
• a first virtual straight line defines a first half and a second half of the distal end face, the first virtual straight line being orthogonal to a virtual second straight line extending from a first center of the first opening, a second center of the second opening being disposed on the second half of the distal end face and the first opening being on a periphery of the first half.