  The decay $\eta'\to\eta\pi^{0}\pi^{0}$ is studied in the framework of isobar
model. It is shown, that good agreement with the experiment is achieved if
$a_0$- and $\sigma$-meson contributions are taken into account. The
contribution of $a_0$-meson is dominant, but $\sigma$-meson is necessary to
reproduce the form of experimental Dalitz plot. Instead of usual Breit-Wigner
form of $\sigma$-meson propagator we use parametrization of $\pi\pi$-amplitude,
that satisfies analiticity, crossing, unitarity and chirality constraints. This
amplitude has a pole in complex plane, that corresponds to $\sigma$-meson and
describe experimental data on $\pi\pi$-scattering in $K_{e4}$ decay.
