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In natural chalcopyrite mineral, Cu_1+xFe_1−xS₂ (x = 0.08), a huge Seebeck effect was reported around the room temperature [R. Ang, et al., Angew. Chem. 54, 1 (2015)]. We study the mechanism of this Seebeck effect and clarify that the observed huge Seebeck coefficient will be due to the antiferromagnetic magnon drag collaborated with the impurity band. First, we show that the temperature dependences of the electrical resistivity and Hall coefficient can be reproduced by the theoretical calculation based on self-consistent t-matrix approximation which can consider the impurity band. Second, by deducing the lifetime of magnon from the experimental result of thermal conductivity, we evaluate the Seebeck coefficient due to the antiferromagnetic magnon drag by the linear response theory of Kubo-Luttinger. We find that the obtained theoretical result is quantitatively consistent with the experimental one near the room temperature.