  Subhorizon approximation is often used in cosmological perturbation theory.
In this paper, however, it is shown that the subhorizon approximation is not
always a good approximation at least in case of $k$-essence model. We also show
that the sound speed given by $k$-essence model exerts a huge influence on the
time evolution of the matter density perturbation, and the future observations
could clarify the differences between the $\Lambda$CDM model and $k$-essence
model.
