We propose to utilize a giant nanomembrane as a substrate for an ultra-thin lightweight device. The giant nanomembrane used in this research is a robust nanometer thick film consisting of highly cross-linked epoxy resins. We fabricated organic thin-film solar cells on the nanomembrane, and monitored the performance of the cells. Successful solar cell fabrication on a nanomembrane by just a sequential lamination of thin films proved that nanomembranes showed enough physical stability, chemical resistance, and surface wettability, to be utilized as a substrate for an ultra-lightweight device. In addition, we tried to detach the nanomembrane solar cell from a solid substrate by dissolving an expendable nanomembrane layer, which had been created solely for this purpose, and scooped it up with our fingers. The total thickness and weight of the detached device were less than 500 nm and 0.8 gm(-2) respectively, which are comparable to the corresponding characteristics of the thinnest and lightest devices in the world. This demonstration will expand the possibility of nanomembranes as substrates for various wearable electronic and optical devices.