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Iron antimonide (FeSb_{2}) with peculiar colossal thermopower of about −45 mV/K at 10 K is a mysterious material, and a unified microscopic description of this phenomenon is far from being achieved. Combining angle-resolved photoemission spectroscopy (ARPES) and ab initio calculations, we find that the intricate electronic structure of FeSb_{2} consists of two bands near the Fermi energy: the weakly dispersing strongly renormalized α band and the holelike β band that intersect at Γ and Y points of the Brillouin zone. In addition, we found the surface state originated from the bulk β band. While both bulk bands upshift towards the Fermi level upon raising of the temperature, the weakly dispersing surface states vanish above 100 K. The structural distortions and/or a mixture of the localized low-spin state with the delocalized high-spin state populated with temperature could be responsible for this temperature dependence. Our study reveals that the sizable renormalization of the nondispersing α band and the hybridization with the holelike β band cause the local increase of the density of states, consequently raising the colossal thermopower in FeSb_{2}.