Vacuum laser welding has been attracting attention as a deep penetration technique for the welding of thick steel plates. It has been reported that this welding method is effective for low spattering and deep penetration in an environment where the atmospheric pressure is reduced to 100 Pa or less. However, it takes time to vacuum, and relaxation of the allowable vacuum is required for application to the production line. In this study, to clarify the laser welding phenomenon in the vacuum condition, the molten pool and plume behavior during welding were observed mainly by the two-color radiation thermometer and the Schlieren method, respectively. A disk laser with a wavelength of 1030 nm was focused through the window of the vacuum chamber. A test material was SUS304 (50×3×50 mm), and the surface of 50×3 mm was laser welded. As the result, the shape of molten pool changed drastically with the passage of time at atmospheric pressure, and the molten metal was extruded from the pool and scattered as spatter when the molten pool shrank. Furthermore, the plume did not erupt at 100 Pa, while the plume erupted periodically from just above the keyhole at atmospheric pressure. It is considered that at atmospheric pressure, the refraction of the laser is caused by the periodically spouted plume and the heat input becomes unstable, which causes the molten pool to repeatedly expand and contract, leading to the generation of spatter.