[en] FP plate-out measurement is being carried out in OGL-1 in order to investigate a fission product behavior in primary circuit of VHTR by a non-destructive method. γ rays penetrating pipe wall are measured using Ge detectors located outside of pipe. It is essential to determine a plate-out conversion coefficient, which converts a measured γ ray peak counting-rate into a plate-out activity, by another method. Conversion coefficient for each measuring point was calculated with detector sensitivity distribution measured and geometrical arrangement, because pipe structures and detector arrangements are different for all points currently measured. In this paper, reported are a technique for measurement of sensitivity distribution, a method for calculation of conversion coefficient, and their results. Besides, error in OGL-1 FP plate-out measurement and propriety of conversion coefficients reported are discussed. (author)