Since electromagnetic (EM) noise resulting from an arc discharge disturbs other electric devices, parameters on electromagnetic compatibility, as well as lifetime and reliability, are important properties for electrical contacts. To clarify the characteristics and the mechanism of the generation of the EM noise, the arc column and voltage fluctuations generated by slowly breaking contacts with external direct current (DC) magnetic field, up to 20mT, was investigated experimentally using AgSnO2 material. Firstly the motion of the arc column is measured by high-speed camera. Secondary, the distribution of the motion of the arc and contact voltage are discussed. Applying the DC magnetic field is effective in suppress the arc duration but voltage fluctuation increases. It was demonstrated that the contact voltage fluctuation in the arc duration is related to the arc column motion.