Since flexible electronic devices can come in close contact with soft human tissue, they are suitable as wearable sensor applications for long-term and non-invasive monitoring of biological signals in a wide range of fields such as medical/health care, sports, and so on. Previous flexible sensor systems require signal processing with either a remote terminal or rigid IC chips placed on the sensor device due to the lack of high-performance flexible analog-frontend circuit (AFE). The remote system may suffer from an incorporation of external noise. The use of rigid IC chips degrades the flexibility and durability of the wearable sensor. To realize a fully flexible sensor system, an AFE including amplifiers, filters, analog-digital converters is needed to be integrated as well as sensors monolithically on a flexible substrate. In this study, we have realized fully-flexible AFE with carbon nanotubes. The key circuit is a novel operational amplifier (op-amp) with a high gain and low operating voltage. The AFE can be operated at low-voltage and exhibits excellent stability and robustness against variability.