A carbon-nanotube (CNT) thin film (Bucky paper) is a promising candidate for a ubiquitous thermoelectric material with large thermoelectric power factor. Recent experiments reported that the temperature dependence and doping dependence of Seebeck coefficient of Bucky papers are essentially different from those of a single CNT. However, such unusual behaviors of Seebeck coefficient of Bucky papers have not been sufficiently understood yet. In this study, in order to clarify the thermoelectric properties of Bucky papers, we introduced a minimal model for Bucky papers in which two CNTs are laterally contacted, and we have performed the first-principles simulation based on the density-functional theory and the none-equilibrium Green's function method.