An endoscope insertion portion characterized by includes an image pick-up assembly (68) extending in an axial direction in the endoscope insertion portion, and including an image pick-up unit (42) and an image pick-up cable (44) provided in order from the distal end portion of the image pick-up assembly (68) to the proximal end side of the image pick-up assembly (68) wherein an outer diameter of the image pick-up unit (42) is larger than an outer diameter of the image pick-up cable (44), and a channel assembly (84) extending in the axial direction in the endoscope insertion portion, arranged side by side with the image pick-up assembly (68), and including a channel pipe (72) and a channel tube (78) provided in order from a distal end portion of the channel assembly (84) to the proximal end side of the channel assembly (84) wherein the channel pipe (72) includes a pipe main portion (74) formed by a distal end side part of the channel pipe (72), the channel tube (78) includes a tube main portion (80) formed by a proximal end side part of the channel tube (78), the channel assembly (84) includes a thin outer diameter portion (86) formed by at least the pipe main portion (74) of the channel pipe (72), and a thick outer diameter portion (88) formed by at least the tube main portion (80) of the channel tube (78) and including an outer diameter larger than an outer diameter of a thin outer diameter portion (86), and a distal end portion of the thick outer diameter portion (88) is arranged on more proximal end side than a proximal end portion of the image pick-up unit (42).
