The upper critical field, Hc2 of β-(BEDT-TTF)2I3 is studied through resistivity measurement under pressure. Because of the sharp angular dependence of Hc2, when the magnetic field is parallel to the a-b plane, care was taken to regulate the angle between magnetic field and the crystal plane. The temperature dependence of the Hc2 was almost linear in the wide range of temperature below the superconducting transition temperature Tc either for the magnetic field parallel or for perpendicular to the two-dimensional plane except when Tc was above 6 K, which was realized when the pressure is around 1.5 kbar for this salt. The value of Hc2 tends strongly to a Pauli limited value(Hp) when the extrapolation of the linear temperature dependence of Hc2 to lower temperature exceeds this value. This fact shows that β-(BEDT-TTF)2I3 is a singlet superconductor.
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