The flute instabilities were investigated in the GAMMA10 A-divertor magnetic field with help of computer simulations. The basic equations used in the simulation can be applied to only an axisymmetric system. So the high pressure in the remaining non-axisymmetric anchor cell, which is used for the flute mode stability, is taken into account by redefining the specific volume of a magnetic field line. It is found that the minimum-B mirror can stabilize a flute mode even in a divertor mirror cell, but its stabilizing effects are weaker. The radial transport accompanied by the flute instabilities in the GAMMA10 A-divertor is found to be rather smaller than that without a divertor mirror cell.