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We present optical and near-infrared observations of the rapidly evolvingsupernova (SN) 2017czd that shows hydrogen features. The optical light curvesexhibit a short plateau phase ($\sim 13$ days in the $R$-band) followed by arapid decline by $4.5$ mag in $\sim 20 \mathrm{days}$ after the plateau. Thedecline rate is larger than those of any standard SNe, and close to those ofrapidly 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 evolvingtransients. The spectra of SN 2017czd clearly show the hydrogen features andresemble those of SNe IIP at first. The H$\alpha$ line, however, does notevolve much with time and it becomes similar to those in SNe IIb at declinephase. We calculate the synthetic light curves using a SN IIb progenitor whichhas 16 M$_{\odot}$ at the zero-age main sequence and evolves in a binarysystem. 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 plateauphase 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 aSN IIb progenitor. We suggest that some rapidly evolving transients can beexplained by such a weak explosion of the progenitors with little hydrogen-richenvelope.

SN 2017czd: A Rapidly Evolving Supernova from a Weak Explosion of a Type IIb Supernova Progenitor