  Until now, Schr\"odinger's cat states are generated by subtracting single
photons from the whole bandwidth of squeezed vacua. However, it was pointed out
recently that the achievable purities are limited in such method (J. Yoshikawa,
W. Asavanant, and A. Furusawa, arXiv:1707.08146 [quant-ph] (2017)). In this
paper, we used our new photon subtraction method with a narrowband filtering
cavity and generated a highly pure Schr\"odinger's cat state with the value of
$-0.184$ at the origin of the Wigner function. To our knowledge, this is the
highest value ever reported without any loss corrections. The temporal mode
also becomes exponentially rising in our method, which allows us to make a
real-time quadrature measurement on Schr\"odinger's cat states, and we obtained
the value of $-0.162$ at the origin of the Wigner function.
