  We derive a simple formula for the width of a multi-channel resonance state.
To this end, we use a deformed square-well potential and solve the
coupled-channels equations. We obtain the $S$-matrix in the Breit-Wigner form,
from which partial widths can be extracted. We apply the resultant formula to a
deformed nucleus and discuss the behavior of partial width for an $s$-wave
channel.
