This paper describes a fast method to fabricate 3D microchannel. We use alginate gel tubes to mold PDMS microchannel. Our method does not need any of those complicated processes such as photolithography or bonding. It consists of three simple steps. (1) Arrangement of the hydrogel tubes according to the objective design, (2) Molding the tubes into PDMS, (3) Dissolution and extrusion of the hydrogel tubes after curing process of the PDMS. The curing process can be performed at normal condition (i.e. 75 degC, 1.5 h). The hollow-centered structure of hydrogel tube reduces fluidic resistance when we dissolved Buried hydrogels in microchannel (Figure 2D). Our method allows us to fabricate complex 3D microfluidics which was difficult to fabricate in the conventional methods using 2D molds fabricated by photolithography.