Dew condensation on metals may cause their corrosion. In order to detect initial condensation, the purpose of this study was to develop small sensor to detect water droplet below 20μm, not detected by current technologies. The sensors, which had two different metal arrays with narrow gap, were made by semiconductor microfabrication technique. Current was measured from sensor, on which arrays were contacted with droplet, bringing about galvanic action. This sensor was succeeded in detecting droplets from 0.5 to 20μm at short response time within 20 msec and with high sensitivity. It had small dimension below 1 mm2, self-power generation and compatibility with silicon technology, leading to full-time and multi-point monitoring of condensation. It was expected to be applied for size distinction of droplet. Micro electrolyte between narrow electrodes could realize easily reaction fields controlled by not diffusion of dissolved oxygen but its electrode reaction, useful for electrochemistry.