In this study, a numerical simulation was carried out to investigate the enhancement of the gas absorption rate caused by the head-on collision of binary droplets. A Lagrangian finite element method was used to analyze the collision of droplets in a stagnant gas containing an absorbed gas species. The effects of Weber number based on the collision velocity of binary droplets and Schmidt number of the absorbed gas species on the gas absorption rate were examined. With increase of Weber number, the gas absorption rate due to the collision of droplets was enhanced up to 4 times that of stationary binary droplets. Furthermore, the enhancement mechanism of the gas absorption was deduced from the velocity and concentration field inside the droplet. During the initial stage of droplet deformation, fresh liquid inside the droplets was supplied to the droplet surface so that the thickness of the mass boundary layer decreases. As a result, the gas absorption is improved. In the latter stage of the deformation, on the other hand, the gas absorption was not enhanced because the concentration field is already developed.