Proton Exchange Membrane Fuel Cells (PEMFC) are accepted to be one of the best technologies that meets the requirements for in-car application. It is very interesting to monitor the impedance of a FC as it gives information on its physical behaviour; it can allow to know whether the membrane is correctly humidified and it can help to understand the electrochemical behaviour of the FC which is important if one wants the FC to operate at its best. We present the experimental device we developed in order to investigate a 5kW fuel cell operating at its nominal current (500 A); it allows to measure the very small value of the impedance (# 10-3 Ω) by exciting the FC with a sinusoidal current. We give experimental results showing the evolution of the impedance on the frequency range: fmin = 1.6 Hz, fmax = 22 kHz. We show the significant evolution of the impedance diagram when the operating current is changed. We present the results we got when the FC membrane is getting dry. In the conclusion, we give some possible investigations to be led thanks to our experimental device.