We have developed a totally flexible, sheet-shaped biofuel cell by stacking a bioanode fabric, a hydrogel sheet containing electrolyte and fuel (fructose), and an O₂-diffusion biocathode fabric. The results presented include two strategies to improve the performance of the device. (1) An anode modified with an appropriate CNT dispersion showed higher activity. (2) The gas-diffusion biocathode was improved by optimizing its hydrophobicity. The improved biofuel cell sheet produced a maximum power density of 1.0 mW cm⁻² at 0.36 V even when bent. Such a flexible, sheet-shaped power source could be combined in the future with flexible electronic to make wearable devices.