We study the
$L^p$
regularity of the Bergman projection P over the symmetrized polydisc in
$\mathbb C^n$
. We give a decomposition of the Bergman projection on the polydisc and obtain an operator equivalent to the Bergman projection over antisymmetric function spaces. Using it, we obtain the
$L^p$
irregularity of P for
$p=\frac {2n}{n-1}$
which also implies that P is
$L^p$
bounded if and only if
$p\in (\frac {2n}{n+1},\frac {2n}{n-1})$
.