An endoscope insertion portion comprising: an image pick-up assembly extending in an axial direction in the endoscope insertion portion, and including an image pick-up unit and an image pick-up cable provided in order from the distal end portion of the image pick-up assembly to the proximal end side of the image pick-up assembly wherein an outer diameter of the image pick-up unit is larger than an outer diameter of the image pick-up cable; and a channel assembly extending in the axial direction in the endoscope insertion portion, arranged side by side with the image pick-up assembly, and including a channel pipe and a channel tube provided in order from a distal end portion of the channel assembly to the proximal end side of the channel assembly wherein the channel pipe includes a pipe main portion formed by a distal end side part of the channel pipe, the channel tube includes a tube main portion formed by a proximal end side part of the channel tube, the channel assembly includes a thin outer diameter portion formed by at least the pipe main portion of the channel pipe, and a thick outer diameter portion formed by at least the tube main portion of the channel tube and including an outer diameter larger than an outer diameter of the thin outer diameter portion, and a distal end portion of the thick outer diameter portion is arranged on more proximal end side than a proximal end portion of the image pick-up unit, wherein the channel pipe includes a pipe fitting portion formed on a proximal end portion of the channel pipe, the channel tube includes a tube fitting portion formed on a distal end portion of the channel tube, the pipe fitting portion is fitted into and fixed to the tube fitting portion, and the thin outer diameter portion is formed by the pipe main portion, the thick outer diameter portion is formed by the tube fitting portion and the tube main portion, an outer diameter and an inner diameter of the channel pipe are substantially fixed along an axial direction of the channel pipe, and an axial length of the tube fitting portion is substantially equal to an axial length of the pipe fitting portion, characterized in that an outer diameter of the channel tube is substantially fixed along an axial direction of the channel tube and larger than the outer diameter of the channel pipe, an inner diameter of the tube fitting portion is substantially equal to the outer diameter of the channel pipe, an an inner diameter of the tube main portion is substantially equal to the inner diameter of the channel pipe. an image pick-up assembly extending in an axial direction in the endoscope insertion portion, and including an image pick-up unit and an image pick-up cable provided in order from the distal end portion of the image pick-up assembly to the proximal end side of the image pick-up assembly wherein an outer diameter of the image pick-up unit is larger than an outer diameter of the image pick-up cable; and a channel assembly extending in the axial direction in the endoscope insertion portion, arranged side by side with the image pick-up assembly, and including a channel pipe and a channel tube provided in order from a distal end portion of the channel assembly to the proximal end side of the channel assembly wherein the channel pipe includes a pipe main portion formed by a distal end side part of the channel pipe, the channel tube includes a tube main portion formed by a proximal end side part of the channel tube, the channel assembly includes a thin outer diameter portion formed by at least the pipe main portion of the channel pipe, and a thick outer diameter portion formed by at least the tube main portion of the channel tube and including an outer diameter larger than an outer diameter of the thin outer diameter portion, and a distal end portion of the thick outer diameter portion is arranged on more proximal end side than a proximal end portion of the image pick-up unit, wherein the channel pipe includes a pipe fitting portion formed on a proximal end portion of the channel pipe, the channel tube includes a tube fitting portion formed on a distal end portion of the channel tube, the pipe fitting portion is fitted into and fixed to the tube fitting portion, and the thin outer diameter portion is formed by the pipe main portion, the thick outer diameter portion is formed by the tube fitting portion and the tube main portion, an outer diameter and an inner diameter of the channel pipe are substantially fixed along an axial direction of the channel pipe, and an axial length of the tube fitting portion is substantially equal to an axial length of the pipe fitting portion, an outer diameter of the channel tube is substantially fixed along an axial direction of the channel tube and larger than the outer diameter of the channel pipe, an inner diameter of the tube fitting portion is substantially equal to the outer diameter of the channel pipe, an an inner diameter of the tube main portion is substantially equal to the inner diameter of the channel pipe.