Energy production and cell division are one of the most important functions for living as a cell. On the other hand, micro algae and protist that are constituted with only one cell have highly diversified functions. There are many phenomena that can not be explained by our current knowledge. For example, delicate siliceous frustule formation in diatoms, rapid haptonematal coiling in haptophytes, predation using a veil in dinoflagellates, shell assembly outside the cell in testate amoebae and so on. Here, we focused on shell assembly of testate amoebae. Cell body of testate amoeba never comes out from its shell except pseudopodia. Before cell division, they form a new shell for one of two daughter cells. Interestingly, testate amoebae assemble scales into round shape using pseudopodia without casting mold outside of their cell. Paulinella chromatophora is one of the few testate amoebae that can be cultivated. We revealed the shell assembly process in P. chromatophora and discussed about the potential function of eukaryotic cell.