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A viewing system was designed and a prototype realized for the in-vessel inspection of the International Thermonuclear Experimental Reactor. The viewing is based on the line scanning principle, and the system consists of 10 identical units installed on top of the reactor at 36 deg intervals. Each device contains a laser, beam steering mirrors, and viewing probe with insertion mechanics. The probe has an outside diameter of 150 mm and a length of 14 m. The illumination design applies frequency-doubled Nd:YAG lasers whose beams are guided through hermetically sealed windows into the vacuum vessel. The diffuser optics creates a vertically oriented light stripe onto the vessel surface that is viewed by the imaging optics, consisting of 16 modules altogether covering horizontal and vertical field-of-views of 2 and 162 degrees. The optical images are transferred to CCD cameras via coherent fiber arrays. The multi-focus design uses stacked fiber rows whose ends are assembled into different axial positions. The viewing probes rotate at a constant angular speed of 1deg/s and pictures are taken at 0.01 deg intervals. The complete picture of the vessel interior is generated in 6 minutes producing 5.8 *10 exp(9) image pixels. The image processing and analysis of possible defects in the vessel surfaces are performed off-line after the viewing procedure. A full-scale prototype of the viewing probe was constructed to demonstrate the feasibility of the design. Its illumination optics utilizes a diffractive optics element that transforms the collimated input beam into a rectangular output lobe with uniform intensity. The prototype has horizontal and vertical imaging optics field-of-views of 2 and 12 degrees. The test results showed that the prototype can take pictures of good quality applying a continuously rotating probe having an angular speed of 0.08 deg/s. In optimum conditions, the minimum resolvable feature size at 3-m distance is smaller than 1 mm, which satisfies the requirement specification. Further development is needed to increase the illumination power to improve the imaging speed and to develop linear fiber arrays that are compatible with the vacuum and high-flux radiation environment of the primary vacuum vessel.