  We present optical and near-infrared observations of the rapidly evolving
supernova (SN) 2017czd that shows hydrogen features. The optical light curves
exhibit a short plateau phase ($\sim 13$ days in the $R$-band) followed by a
rapid decline by $4.5$ mag in $\sim 20 \mathrm{days}$ after the plateau. The
decline rate is larger than those of any standard SNe, and close to those of
rapidly evolving transients. The peak absolute magnitude is $-16.8$ mag in the
$V$-band, which is within the observed range for SNe IIP and rapidly evolving
transients. The spectra of SN 2017czd clearly show the hydrogen features and
resemble those of SNe IIP at first. The H$\alpha$ line, however, does not
evolve much with time and it becomes similar to those in SNe IIb at decline
phase. We calculate the synthetic light curves using a SN IIb progenitor which
has 16 M$_{\odot}$ at the zero-age main sequence and evolves in a binary
system. The model with a low explosion energy ($5\times 10^{50}$ erg) and a low
${}^{56}$Ni mass ($0.003 \mathrm{M}_{\odot}$) can reproduce the short plateau
phase as well as the sudden drop of the light curve as observed in SN 2017czd.
We conclude that SN 2017czd might be the first identified weak explosion from a
SN IIb progenitor. We suggest that some rapidly evolving transients can be
explained by such a weak explosion of the progenitors with little hydrogen-rich
envelope.
