De Haas-van Alphen (dHvA) measurements have been performed on the quasi two dimensional monophosphate tungsten bronze (PO2)(4)(WO3)(2m) (m=6) in magnetic fields B up to 28T by a capacitive torquemeter. The induced gap opening in the charge density wave state (CDW) of this inorganic conductor reduces the Fermi Surface (FS) to small pockets and magnetic breakdown is favoured between orbits on neighbouring pockets. dHvA measurements in the (100) plane reveal two extremal orbits of two cylindrical FS with frequencies F-alpha=76T, F-beta=102T (B parallel to c), and one possible magnetic breakdown orbit (F-alpha+beta=180T) between these two orbits.