This paper examines the possibility of applying the time‐dependent perturbation theory to the systems with a time‐dependent unperturbed part. Analyses indicate that time‐dependent perturbation theory is valid even if the unperturbed HamiltonianH0depends on time, provided that the pseudostationary states ofH0are known. It is also shown that this new perturbative treatment provides a useful tool for tackling the problem of a general driven time‐dependent oscillator.