Flux-gate magnetometer has been still used for low field magnetic field measurement with portability, low power consumption, and high reliability. In many applications, flux-gate magnetometer measures not absolute values but changes of the earth magnetic field. For the eia magnetic field change measurements, we have constructed a high sensitive 3-axis flux-gate magnetometer of which measuring ranges is ${pm}$1000 nT and noise level is 5pT/√㎐ at 1 ㎐. Using this magnetometer, we can compensate the earth magnetic field of ${pm}$50,000 nT with successive approximation methods using microcomputer. After earth magnetic field compensation, we could measure earth magnetic field changes with ${pm}$100 nT measuring ranges.