  We consider the spectral structure of the Neumann--Poincar\'e operators
defined on the boundaries of thin domains of rectangle shape in two dimensions.
We prove that as the aspect ratio of the domains tends to $\infty$, or
equivalently, as the domains get thinner, the spectra of the
Neumann--Poincar\'e operators are densely distributed in the interval
$[-1/2,1/2]$.
