Piton de la Fournaise is a highly active basaltic volcano located on La Réunion island. With the aim of gaining information on its 3D internal structure, we carried out a 3D tomographic inversion based on P-wave first arrival times for earthquakes recorded by the local seismic monitoring network of the Observatoire Volcanologique du Piton de la Fournaise (OVPF).We used data recorded between 1996 and 1999 during intereruptive periods as well as during the pre-eruptive swarms which preceded the eruptions in March 1998, July and September 1999. Volcano-tectonic activity below the volcano is mostly located at shallow depth, above sea level and below the central cone. However, the seismic crisis which preceded the March 9, 1998 eruption included a large number of events at greater depth providing an unprecedented data set which sheds light on the deep structure of the volcano. The tomographic technique which we used is based on an accurate finitedifference travel-time computation and a simultaneous probabilistic inversion of both velocity models and earthquake locations. The inversion is carried out using an improved technique which allows to well constrain the inversion parameters. This processing provides high quality stable tomographic images. The obtained P-wave velocity model confirms the presence of a high-velocity plug above sea level, under the summit craters. This anomaly is interpreted as corresponding to an intrusive, solidified dyke-and-sill complex with little fluid magma storage. The high number of hypocenters in the zone above sea level accounts for massive rock fracturing when magma rises toward the surface. The shallow high-velocity plug is surrounded by a lowvelocity ring known as being fractured and vesicular lavas or scorias located on volcano flanks. Two low-velocity anomalies are found below the summit caldera. The most superficial spreads from 1 km to 0 a.s.l.. The second is located below 1 km b.s.l.. These volumes may correspond to magma storage systems. They are separated by a relatively high-velocity volume at sea level with a strong velocity gradient reaching 0.6 km/s per km