Laser metal deposition is a process by which powder particles sprayed together with a gas are melted and solidified on a based metal by a laser beam. This process is attracting attention not only for repair and surface modification, but also for additive manufacturing. Clad beads sometimes have pore defects that deteriorates corrosion resistance and mechanical properties. However, the mechanism of pore defect generation has not been clarified. In this study, a direct observation technique using two quartz glass plates was developed to understand the bubble generation processes inside a molten pool, which are the causes of pore defects. This direct observation technique revealed the mechanical or chemical bubble generation process depending on a powder material. For Ni based super alloy powder, the cavities were formed as the powder particles rushed into the molten pool. Then, a part of the cavity remained in the molten pool as bubble. For WC-12Co powder, the bubbles were generated mainly from inside of the molten pool. This result suggested that gas was continuously generated from the molten pool during laser metal deposition, unlike the case of Ni-based super alloy powder. Therefore, in order to reduce pore defects, an optimal approach is required depending on the powder material.