Exotic surface states of topological insulators have long attracted the attention of researchers. Recently, surface-dominant electrical transport in topological insulators has been observed; however, surface conduction in topological insulators is still not fully understood. To address this knowledge gap, we measured the transport properties of a thin flake of a highly bulk-resistive topological insulator, Sn0.02Bi1.08Sb0.9Te2S (Sn-BSTS), whose carrier density was controlled with the field effect. Single crystals of Sn-BSTS were synthesized by the Bridgman method, and Hall devices were fabricated with exfoliated flakes. The bottom gate structure was used to control the bottom surface of a Sn-BSTS flake. The measured Hall resistance was analyzed using the two-band model, which quantitatively showed that ambipolar conduction was achieved. In addition, the carriers on the top surface were controlled by the formation of an electrical double layer by an ionic liquid. With a top-gate voltage of -1.5 V, a massive number of p-type carriers were induced on the top surface of the Sn-BSTS flake, as also confirmed with the two-band model. The longitudinal resistance was also found to be affected by the carrier density. The magnetoresistance was enhanced when n- and p-type carriers coexisted on the top and bottom surfaces. In particular, the magnetoresistance was quantitatively shown to increase when the densities of n- and p-type carriers were similar. This study is the first to quantitatively analyze the conduction in Sn-BSTS in the presence of multiple types of carriers. Our findings pave the way for a quantitative understanding of transport phenomena in topological insulators.