The dust mobilization in the vacuum vessel under the Loss-of-Vacuum-Accident (LOVA) event was measured quantitatively using the preliminary LOVA apparatus. The particle size distributions in the mobilized dust were characterized and analyzed using a high performance optical-microscope and image analysis software. It was found that around 10% of the initial dust weight was transported upward inside the VV when the breach located at the roof of the VV and an incoming flow from the outside through the breach directly hit the dust. On the other hand, the transported dust weight was less than 1% of the initial dust weight when the breach position was at the side wall of the VV and an incoming flow from the outside indirectly hit the dust. The relationship between the dust mobilization and breach position was clarified from the present experimental results.