  Diffusivity in some soft matter and biological systems changes with time,
called the fluctuating diffusivity. In this work, we propose a novel origin for
fluctuating diffusivity based on stochastic simulations of binary gas mixtures.
In this system, the fraction of one component is significantly small, and the
mass of the minor component molecule is different from that of the major
component. The minor component exhibits fluctuating diffusivity when its mass
is sufficiently smaller than that of the major component. We elucidate that
this fluctuating diffusivity is caused by the time scale separation between the
relaxation of the velocity direction and the speed of the minor component
molecule.
