  The dressed state formalism enables us to define the infrared finite
$S$-matrix for QED. In the formalism, asymptotic charged states are dressed by
clouds of photons. The dressed asymptotic states are originally obtained by
solving the dynamics of the asymptotic Hamiltonian in the far past or future
region. However, there was an argument that the obtained dressed states are not
gauge invariant. We resolve the problem by imposing a correct gauge invariant
condition. We show that the dressed states can be obtained just by requiring
the gauge invariance of asymptotic states. In other words, Gauss's law
naturally leads to proper asymptotic states for the infrared finite $S$-matrix.
We also discuss the relation between the dressed state formalism and the
asymptotic symmetry for QED.
