Abstract : he absolute measurement of the Photo Detection Efficiency (PDE) of a SiPM over the whole visible range using continuous light is not a straightforward task as it requires the accurate determination of the detector gain. The SiPM gain given by the producer does not take into account all the secondary effects (cross-talks, after-pulses events), which leads to an overestimation of the PDE. To measure the absolute SiPM PDE, we have carried out at LAL two kinds of measurements following what we call "the "Counting method" and the "Photocurrent method". In the "counting method", the very low number of photons in the short light pulses is Poisson distributed which allows us to count the photoelectrons produced. With this method, we do not need to consider the SiPM gain and the PDE is measured with an accuracy of 5 %. With the "Photocurrent method", the SiPM is illuminated by continuous light and the number of converted photons is determinate measuring the generated photocurrent and the gain. We detail these two methods and compare their results.