  Extraction of multiple quasinormal modes from ringdown gravitational waves
emitted from a binary black hole coalescence is a touchstone to test whether a
remnant black hole is described by the Kerr spacetime in general relativity.
However, it is not straightforward to check the consistency between the
ringdown signal and the quasinormal mode frequencies predicted by the linear
perturbation theory. While the longest-lived mode can be extracted in a stable
manner, the higher overtones damp more quickly and hence the fitting of
overtones tends to end up with the overfit. To improve the extraction of
overtones, we propose an iterative procedure consisting of fitting and
subtraction of the longest-lived mode of the ringdown waveform in the time
domain. Through the analyses of the mock waveform and numerical relativity
waveform, we clarify that the iterative procedure allows us to extract the
overtones in a more stable manner.
