  We perform a high-cadence transient survey with Subaru Hyper Suprime-Cam
(HSC), which we call the Subaru HSC survey Optimized for Optical Transients
(SHOOT). We conduct HSC imaging observations with time intervals of about one
hour on two successive nights, and spectroscopic and photometric follow-up
observations. A rapidly declining blue transient SHOOT14di at $z=0.4229$ is
found in observations on two successive nights with an image subtraction
technique. The rate of brightness change is $+1.28^{+0.40}_{-0.27}~{\rm
mag~day^{-1}}$ ($+1.83^{+0.57}_{-0.39}~{\rm mag~day^{-1}}$) in the observer
(rest) frame and the rest-frame color between $3400$ and
$4400~\unicode[.8,0]{x212B}$ is $M_{\rm 3400\unicode[.8,0]{x212B}}-M_{\rm
4400\unicode[.8,0]{x212B}}=-0.4$. The nature of the object is investigated by
comparing its peak luminosity, decline rate, and color with those of transients
and variables previously observed, and those of theoretical models. None of the
transients or variables share the same properties as SHOOT14di. Comparisons
with theoretical models demonstrate that, while the emission from the cooling
envelope of a Type IIb supernova shows a slower decline rate than SHOOT14di,
and the explosion of a red supergiant star with a dense circumstellar wind
shows a redder color than SHOOT14di, the shock breakout at the stellar surface
of the explosion of a $25M_{\odot}$ red supergiant star with a small explosion
energy of $\leq0.4\times10^{51}$ erg reproduces the multicolor light curve of
SHOOT14di. This discovery shows that a high-cadence, multicolor optical
transient survey at intervals of about one hour, and continuous and immediate
follow-up observations, is important for studies of normal core-collapse
supernovae at high redshifts.
