Fe-Si sheet, which has {110}<001> crystal orientation (Goss-FeSi), consists of Basic domain and Lancet-comb domain. Although the geometric structures of the Lancet-comb domain have been predicted by theoretical aspects, they have not been proved by experimental studies.In this paper, we study the Lancet-comb domain structures of a Fe-Si steel by a volume sensitive method, i.e., transparent X-ray topography (XRT). From the XRT results, we found the parallelogram structures, which is bulk structure of the Lancet-comb domain. The long side of the parallelogram forms about 55° angle to the rolling direction and the short side forms about 35° angle to the rolling direction. We assigned the long side of the parallelogram to {110} domain wall and the short side to {111} domain wall based on the geometric analysis. This Lancet-comb domain model well explains the behavior of the magnetic moments of Goss-FeSi.