[en] We discovered the 3-dimensional twist motion of sperm flagella of the stag beetle Prosopocoilus inclinates. The morphological features are discussed with experimental data obtained through various 'imaging techniques' including those developed in thermo-nuclear fusion research. The helical deformation length observed in the optical micrograph agreed statistically with those of transmission electron micrographs (TEM) on both ultra-thin section and negatively stained samples. This indicated that the helical twist mechanism of flagellar axoneme could be safely discussed from TEM. In order to elucidate this, we applied the newly developed Constrained Electron Beam Tomography (CEBT) technique adapted from our unique fusion plasma diagnosis. This requires basic assumptions of 'the optimum deformation' and 'the coherent length' as mathematical constraints. The results are the key parameters of the flagellum deformation, e.g. the helical pitch (HP) of both axoneme and mitochondrial derivatives as well as the phase slip (PS) between them. They allow the quantitative discussion on this motion. (author)