Abstract: An amperometric sensor device comprises in combination a sensor (21) having a planar structure obtained by photolithographic techniques and a circuit (22) for measuring the intensity of the electrochemical current generated by the sensor (21). The planar structure has an insulating substrate (23 to 25) and a set of electrodes composed of a working electrode (29), a counter electrode (30) and a reference electrode (31). A diffusion membrane (32) is deposited on the working electrode (29). Connection means (26) are provided to connect the electrodes (29,30,31) to the measuring circuit (22). According to the invention, the active conducting part of the working electrode (29), situated at the surface of the substrate (23) is entirely covered by the diffusion membrane (32) which overlaps this electrode (29) over its entire peripheral zone, whereas this membrane is entirely exposed so that, during the operation of the device (20) it is in contact with the fluid to be analysed over its entire surface.