We investigate the scattering of a plane wave by a rectangular dielectric cylinder situated in the free space and consider a specific case when the cylinder is transformed to a thin cylindrical dielectric strip of a rectangular cross section. The cylinder is filled with a homogenous isotropic medium. The analysis is reduced to a boundary value transmission problem (BVTP) for the Helmholtz equation with a piecewise constant coefficient in an unbounded domain. By an integral representation of the scattered field, the solution to the problem is defined via superpositions of the single-layer and double-layer potentials. Then BVTP is reduced to a system of boundary integral equations (IEs) of a mixed type. A detailed analysis of the kernel matrix function is performed and its singularity is separated. A transition to the limiting case is considered when the cylinder thickness tends to zero (and the plate is transformed to an infinitely thin material strip) and the corresponding IE system is derived and analyzed. A numerical method is developed together algorithms and MATLAB codes and three model problems of the scattering from dielectric plates of decreasing thickness is solved numerically.