Recently, atmospheric carbon dioxide concentration, which may enhance on-going global warming issue, is increasing rapidly. Here, methanation of atmospheric CO₂ has been suggested. However, in the methanation process developed so far, at least 200-300 °C is required with using catalysts like Ni. Therefore, it is desirable to reduce the reaction temperature by different processes from the viewpoint of energy saving and also to avoid deactivation of catalysts at high temperatures. We have recently reported that mechanochemical process of La-Ni based alloys under the presence of CO₂ and H₂ generates CH₄ successfully. But, it has also been suggested by Atom Probe Tomography (APT) analysis carried out in this study, that CO₂ might have partially reacted with the LaNi₅ powder in the course of ball-milling, yielding different La-oxides, carboxides and hydrocarboxylic species containing La embedded in Ni matrix. These compounds might have acted as effective intermediate phases for the methanation reaction. In order to elucidate the origin of the catalytic activity, in-situ monitoring of the pressure change during the process is necessary; reactivity of CO₂ with LaNi₅ in the said process should be investigated as well. Herein, monitoring results of mechanochemical methanation reaction by using vibrational ball-mill are presented.