[en] High burn-up in a core is one of key issues to reduce running cost of a sodium cooled fast reactor. It may result in fuel pin deformation due to irradiation, e.g., swelling and bowing. Such deformation will influence thermal hydraulics in a core fuel subassembly, i.e., the highest temperature. Thus it is significant to estimate pin deformation effects. A water experiment was carried out to measure velocity field in a deformed pin bundle by using transparent resin and refractive index matching technique. In the experiment, the deformed pin geometry should be measured as a boundary condition. Here an optical measurement technique of the pin bundle geometry was developed. An image of a horizontal cross section of the pin bundle was captured by a camera set on an inclined line to the pin bundle axis. This image was converted to a straight image to the pin bundle axis by taking accounts of refraction at the wall. The positions of pins were analyzed based on the converted image. As a basic verification, an illustration sheet, where cross sectional view of fuel pins was drawn, was set in the hexagonal wrapper tube filled with water. The optical measurement method was applied this geometry. The obtained pin positions were in good agreement with the original positions in the sheet. The accuracy of the pin positions was less than one pixel of camera (CCD: 1,000 x 1000 pixels). This image conversion method was applied to the velocity measurement in a horizontal cross section in a 7 pin bundle using PIV (Particle Image Velocimetry). Applicability to the velocity measurement was shown. (author)