Temperature dependence of magnetoresistance in the range of 2—300K and the Δρ /ρ０-B relation at 2K were measured in quasi-two dimensional charge- density wave compound (PO2)4(WO3)2m( m=6). The enhancement of the magnetoresistance at low temperatures was analyzed by use of the magnetic breakdown model. The first Peierls energy gap in the char ge-density wave states was estimated to be about 3.0meV.