  The relationship between charge density wave (CDW) orders and
superconductivity in arsenide superconductor SrPt$_2$As$_2$ with $T_c$ = 5.2 K
which crystallizes in the CaBe$_2$Ge$_2$-type structure was studied by
$^{75}$As nuclear magnetic resonance (NMR) measurements up to 520 K, and
$^{75}$As nuclear quadrupole resonance (NQR) and $^{195}$Pt-NMR measurements
down to 1.5 K. At high temperature, $^{75}$As-NMR spectrum and nuclear spin
relaxation rate ($1/T_1$) have revealed two distinct CDW orders, one realized
in the As-Pt-As layer below $T_{\rm CDW}^{\rm As(1)}$ $=$ 410 K and the other
in the Pt-As-Pt layer below $T_{\rm CDW}^{\rm As(2)}$ $=$ 255 K. The $1/T_1$
measured by $^{75}$As-NQR shows a clear Hebel-Slichter peak just below $T_c$
and decreases exponentially well below $T_c$. Concomitantly, $^{195}$Pt Knight
shift decreases below $T_c$. Our results indicate that superconductivity in
SrPt$_2$As$_2$ is in the spin-singlet state with an $s$-wave gap and is robust
under the two distinct CDW orders in different layers.
