Magnetic Field measurement (MGF) is desigened to measure the magnetic field around the Mars. The magnetic environment of the Mars is still unknown. Recently American spacecraft,Mars Global Surveye r, has been sent there and found that Mars has only the strong local magnetic field and seems to hav e no global intrinsic field. But there are still controversial arguments about the magnitude and str ucture of the Martian magnetic field ,because the pressure due to the intrinsic magnetic field shoul d do exist when you think about the pressure balance between the solar wind plasma and Martian atmos phere in the dayside of Mars. This Planet-B mission includes low altitude measurements of Martian at mosphere and can contribute to the resolution of the unsolved ploblems.