We propose a wide-field deep HSC photometry of the halo region of M33. M33 is relatively well observed in the central region, but few deep observations have been conducted in the outer region. Therefore, the existence and nature of the M33 stellar halo and its halo substructures are still under debate. The combination of the narrow-band filter NB515 and 3 broad-band filters g, r, i makes it possible to detect the faint structures of M33 without contaminations. Combining the NB515 with g and i bands efficiently separates red giant branch stars in M33 from foreground dwarf stars, while the combination of g, r, and i bands discriminates background unresolved galaxies. With the widest field-of-view among 8m-class telescopes, Subaru/HSC covers the outer region of M33 and can detect horizontal branch and red clump stars at greater depths. We make use of the data to detect the M33 stellar halo and faint halo substructures, investigate the nature of its halo, confirm stellar populations that are lower metallicity than large galaxies, and compare the surface brightness profile with those of other spiral galaxies, providing new insights into galactic halos. This observation unravels the M33 stellar halo and reveals global properties of M33 stellar halo for the first time.
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