An endoscope comprising an insertion part inserted into the body, wherein the insertion part includes an outer tube that forms an outer peripheral wall of the insertion part, an optical member disposed at a distal end of the insertion part, an imaging device that is configured to pick picks up an observation image obtained through the optical member, a first signal line that has a distal end connected to the imaging device and is configured to transmit a signal output from the imaging device, a signal relay part connected to the proximal end of the first signal line, a second signal line that has a distal end connected to the signal relay part and is configured to transmit a signal relayed from the first signal line via the signal relay part, a partition wall part that holds the signal relay part, and an airtight casing that is disposed inside the outer tube, has a distal end, a proximal end, and a longitudinal axis, and has a tubular shape of which the inside is hollow, the distal end being airtightly sealed by the optical member, the proximal end being airtightly sealed by the partition wall part, and the imaging device and the first signal line being accommodated in an airtight state therein, wherein the airtight casing includes a first tubular body having a first holding part that holds the optical member and comprising a distal-end-side tubular body having a a fixing part that fixes the imaging device to the inside thereof and a second tubular body having a second holding part that holds the partition wall part, wherein the second tubular body has a second wall surface that faces a first wall surface of the first tubular body, wherein an internal diameter on a proximal end side of the first tubular body is larger than an external diameter on a distal end side of the second tubular body, wherein the first wall surface is an inner wall surface of the first tubular body and the second wall surface is an outer wall surface of the second tubular body, wherein the outer wall surface of the second tubular body is in sliding contact with the inner wall surface of the first tubular body in an airtightness held state. an insertion part inserted into the body, wherein the insertion part includes an outer tube that forms an outer peripheral wall of the insertion part, an optical member disposed at a distal end of the insertion part, an imaging device that is configured to pick picks up an observation image obtained through the optical member, a first signal line that has a distal end connected to the imaging device and is configured to transmit a signal output from the imaging device, a signal relay part connected to the proximal end of the first signal line, a second signal line that has a distal end connected to the signal relay part and is configured to transmit a signal relayed from the first signal line via the signal relay part, a partition wall part that holds the signal relay part, and an airtight casing that is disposed inside the outer tube, has a distal end, a proximal end, and a longitudinal axis, and has a tubular shape of which the inside is hollow, the distal end being airtightly sealed by the optical member, the proximal end being airtightly sealed by the partition wall part, and the imaging device and the first signal line being accommodated in an airtight state therein, wherein the airtight casing includes a first tubular body having a first holding part that holds the optical member and comprising a distal-end-side tubular body having a a fixing part that fixes the imaging device to the inside thereof and a second tubular body having a second holding part that holds the partition wall part, wherein the second tubular body has a second wall surface that faces a first wall surface of the first tubular body, wherein an internal diameter on a proximal end side of the first tubular body is larger than an external diameter on a distal end side of the second tubular body, wherein the first wall surface is an inner wall surface of the first tubular body and the second wall surface is an outer wall surface of the second tubular body, wherein the outer wall surface of the second tubular body is in sliding contact with the inner wall surface of the first tubular body in an airtightness held state.