We have investigated thermoelectric properties of a three-dimensional Rashba system BiTeI. Magnetic-field dependences of the Seebeck effect and Nernst effect show qualitative changes with the Fermi level passing through the bulk Dirac point, indicating that thermoelectric effects can be a good experimental probe for the Fermi surface topology. The quantum oscillations are observed in the thermoelectric effect of BiTeI under a magnetic field, which are also dependent on the Fermi-level positions and consistent with the energy derivative of the three-dimensional density of states in the Rashba system. Emergence of exotic electronic band structures by strong spin-orbit interaction (SOI) is of great interest in condensedmatter physics. In systems without inversion symmetry, a SOIdriven effective magnetic field causes the spin splitting in the electronic band structure except at the high-symmetry points in the Brillouin zone, generating the peculiar spin-polarized linear band dispersion. Among them, the Rashba-type SOI [1] in a polar system generates a spin-polarized Dirac fermion; a SOI-induced k-dependent Zeeman field perpendicular to the polar axis and crystal momentum give rise to the chiral spin texture of the electronic band. The Rashba effect has been conventionally studied at the surface [2-4] and interface [5], but recently, a bulk polar semiconductor BiTeI has been proven to host the large Rashba-type SOI [6]. The Rashba-type band structure is unique in the sense that a small inner Fermi surface (IFS) with a spin-polarized Dirac fermion is buried in the large outer Fermi surface (OFS) also with the chiral spin texture. Transport properties originating from the Rashba-type band structure have been reported in the previous studies, such as the characteristic Shubnikov-de Haas (SdH) oscillations [7][8][9][10][11][12] and magnetophotocurrent measurement [13].A recent systematic study of SdH oscillations in BiTeI [11] could capture the signature of the Fermi surface topology change. With increasing the mobility and varying the carrier density by Cu doping (see also Ref. [12]) the SdH oscillations were observed for the crystals with different Fermi-level positions. An observed qualitative difference of the SdH oscillations between E F E D and E F E D was explained by the scattering process reflecting the three-dimensional (3D) density of sates (DOS) of the IFS in this bulk Rashba system. The purpose of this study is to investigate thermoelectric response (Seebeck and Nernst effects) under a magnetic field in BiTeI with the Fermi level varied around the Dirac point.Thermoelectric effects have been known as a sensitive experimental tool for probing the electronic states in solids, although they are still less understood theoretically. Quantum oscillations in the thermoelectric effects, for example, were used to study the Landau levels of the electronic band structures in Bi [14,15], graphene [16,17], graphite [18], and various topological insulators [19][20][21]. The anomaly in the quantum limit [16,17] or during t...