A micro-plasma device is used as a gas sensor. The micro-plasma device produces dc discharges with a parallel-plane electrode configuration. In this paper the breakdown voltage is measured for argon with tin oxide electrodes, for three different electrode distances (0.01 cm, 0.025 cm and 0.05 cm) and for pressure between 1.3 kPa and 13.3 kPa. The effective secondary electron emission coefficient is then determined from breakdown voltage results; its behaviour is analysed and compared with other experimental results. Differences between experimental breakdown voltage and a widely accepted analytical form of the breakdown law are also shown and the influence of the secondary electron emission coefficient is underlined.