Tubules are fundamental units of our bodies and all metazoans. Its formation in organoid cultures is still imperfect in general. Axial elongation and uniform inner diameter are two geometrical characteristics of tubules. While tubulation and elongation are known to be controlled by external cues, how inner diameter is regulated is poorly understood. In order to gain deep understanding of tubule formation mechanisms, we use tracheal tubulogenesis in Drosophila as a model system. We address the structural basis of materials inside of the tube lumen and mechanical cross talk between luminal matrix and tubule epithelial cells to establish tube geometry.