<p>Metal additive manufacturing has been applied to produce products in various industrial fields such as aerospace, medical and so on because it enables the integrated manufacturing of complex-shaped products with the addition of new functions. However, because generation of defects is possible owing to the intrinsic properties of metal laser powder bed fusion (PBF-LB/M), the development of an in-process monitoring and feedback control technology is demanded to assure the final product quality and process repeatability. In this study, an in-situ monitoring system capable of simultaneously measuring the surface textures of the powder bed and built part and investigating the melting phenomena was developed. The surface textures of the powder bed and built part were able to be quantified by introducing a parameter of 2σ which is nearly equal to the areal surface texture parameter of Sq. It was elucidated that the shape of the melt pool during multi-track scanning was asymmetric in the scanning direction, and spattering occurs excessively toward the built part side because the vapor plume direction turns to the built part side due to the asymmetric melt pool. Moreover, it was revealed that there is a strong correlation between the areal surface-texture parameters and density or internal defects. Consequently, the systematic understanding of the PBF process through the quantification of the surface texture of the built part and the consideration of melt pool behavior leaded to the development of the in-process monitoring and feedback control system for PBF machines.</p>