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We have performed electrical resistivity measurements of a polycrystalline sample of FeSe 0.25 Te 0.75 , which exhibits superconductivity at T c ∼ 14 K, in magnetic fields up to 55 T to determine the upper critical field µ 0 H c2 . In this compound, very large slopes of µ 0 H c2 at the onset, the mid-point, the zero-resistivity temperatures on superconductivity are determined to be −13.7, −10.1, and −6.9 T/K, respectively. The observed µ 0 H c2 (T )s of this compound are considerably smaller than those expected from the Werthamer-Helfand-Hohenberg model, manifesting the Pauli limiting behavior. These results suggest that this compound has a large Maki parameter, but it is smaller than that calculated for a weakcoupling superconductor, indicating a large superconducting gap of this compound as a strong-coupling superconductor.Recent discovery of superconductivity at T c = 26 K in the iron-based LaFeAsO 1−x F x 1 (abbreviated as the 1111-system) has attracted a considerable attention of condensed matter scientists due to an unusual interplay of superconductivity and magnetism arising from ubiquitous magnetic element Fe. In general, it has been believed that materials containing the magnetic elements are difficult to occur the superconductivity. By substitution of La atoms for other lanthanoid atoms, T c increases with increasing lanthanoid atomic number and shows a maximum value of T c = 55 K for Sm atoms. As well as the CuO 2 layers in the high-T c cuprate superconductors, the Fe-P n (P n = P, As) layers in iron oxypnictides are responsible for the superconductivity, and the Ln-O (Ln = lanthanoid) layers provide charge carriers.The electrical structure of a high-T c cuprate superconductor has been explained by the single band model, but it is difficult to understand the mechanism of its superconductivity due to the strongly electron correlation effects. While the electron correlation on an iron-based su- *

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