The black hole X-ray binary 4U 1630$-$472 shows outbursts repeatedly with an interval of about 600 days.
We studied the 2012 outburst, which was one of the biggest outbursts of the source.
We found unusual X-ray flux drops by $\sim 50\%$ with a duration of $\sim 1$ day at its brightest phase,
which is much shorter than the accretion timescale but longer than the dynamical timescale
in the inner region of the accretion disk.
To understand the cause of the drops, we compared the Swift and MAXI spectra obtained
during and outside the drops.
We found that, during the flux drops, the source decreased its flux without significantly
changing the spectral shape.
We consider a possible interpretation for the observed spectral variation:
fully ionized, Compton-thick gas passed through our line of sight and reduced
the apparent X-ray flux.