To determine the epoch of reionization precisely and to reveal the property of inhomogeneous reionization are some of the most important topics of modern cosmology. Existing methods to investigate reionization which use cosmic microwave background, Ly$alpha$ emitters, quasars, or gamma ray bursts, have difficulties in terms of accuracy or event rate. We propose that recently discovered fast luminous blue transients (FLBTs) have a potential as a novel probe of reionization. We study the detectability of FLBTs at the epoch of reionization with upcoming WFIRST Wide-Field Instruments (WFI), using a star formation rate derived from galaxy observations and an event rate of FLBTs proportional to the star formation rate. We find that if FLBTs occur at a rate of 1% of the core-collapse supernova rate, 2 (0.3) FLBTs per year per deg$^2$ at $z>6$ ($z>8$) can be detected by a survey with a limiting magnitude of 26.5 mag in the near-infrared band and a cadence of 10 days. We conclude that the WFIRST supernova deep survey can detect $sim20$ FLBTs at the epoch of reionization in the near future.
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