A method to measure magnetic field fluctuation using a heavy ion beam probe for a toroidal magnetically confined plasma is proposed by providing a mathematical formula to connect the probing beam movement with the change in the magnetic field. The. rst challenge of the method has been faced in an electron-cyclotron resonance heated plasma in the compact helical system (CHS). The results reveal the existence of three characteristic frequency regimes of the magnetic field fluctuations: a low frequency regime showing a long-distance correlation, an intermediate frequency regime obeying a power law and the turbulence regime. Discussion is held on the degree of the contaminations in the local magnetic field fluctuation that are caused by the magnetic field fluctuations integrated along the beam orbit and the local electric field fluctuations.