  We report the detection of a dwarf satellite candidate (Triangulum IV: Tri
IV) of the Triangulum galaxy (M33) using the deep imaging of Subaru/Hyper
Suprime-Cam (HSC). From the apparent magnitude of the horizontal branch in Tri
IV, the heliocentric distance of Tri IV is estimated to be $932^{+49}_{-43}$
kpc, indicating that Tri IV is located at the distance of $75^{+48}_{-40}$ kpc
from the M33 center. This means that Tri IV is the probable satellite of M33,
because its distance from M33 is within the virial radius of M33. We also
estimate its surface brightness of $\mu_{\it V} = 29.72^{+0.10}_{-0.10}$ mag
arcsec$^{-2}$, and half-light radius of $r_h = 1749^{+523}_{-425}$ pc,
suggesting that Tri IV is an ultra-diffuse galaxy or dynamically heated galaxy.
The surface brightness of Tri IV is too low to be detected in the previous
survey, so this detection suggests that much fainter satellites may be present
in the outskirts of M33.
