We developed a digital 3-axis flux-gate magnetometer for magnetic field signal measurement from warship during demagnetizing and degaussing processes. For the magnetometer design, we considered following points; the distance between magnetic field measurement station and magnetometer located under sea is about several 100 m, the magnetometer is exposed to magnetic field of ${pm}1mT$ during demagnetizing process, and magnetometer is located under the sea about 30 m depth. To overcome long distance problem, magnetometer could be operated on wide input supply voltage range of 16~36 V using DC/DC converter, and for the data communication between the magnetometer and measurement station a RS422 serial interface was employed. To improve perming effect due to the ${pm}1mT$ during demagnetizing process, magnetometer could be compensated external magnetic field up to ${pm}1mT$ but magnetic field measuring rang is only ${pm}100{mu}T$. The perming effect was about ${pm}2nT$ under ${pm}1mT$ external magnetic field. The magnetometer was tested water vessel with air pressure up to 6 bar for the sea water pressure problems. Linearity of the magnetometer was better than 0.01 % in the measuring range of ${pm}0.1mT$ and noise level was $30pT/sqrt{Hz}$ at 1 Hz.