It is well known that porosity is easily formed in high power laser welding, which is quite a serious problem to be solved. At present, there are few reports studying interrelationship between keyhole and plasma behavior with the objective of understanding the effect of shielding gas on porosity formation. In this study, therefore, the relationship between keyhole and plasma behavior was observed directly by using two synchronized ultra high-speed cameras and X-ray transmission observation system. In the case of He gas, metallic plasma was continuously formed, and the keyhole was always open. It was observed that many large bubbles, which were formed from the tip of a keyhole, were trapped at the solidifying front in the rear part of the molten pool, and lead to the porosity formation. On the other hand, in the case of N2 gas, big nitrogen plasma was formed above the weld bead periodically, and its absorption of laser caused the disappearance of metallic plasma and keyhole. This periodical interval and duty were different among materials used and affected the bubble and porosity suppression beneficially.