We present observations of the four hyperfine structure components of the OH 18 cm transition (1612, 1665, 1667 and 1720 MHz) toward a filamentary dark cloud, the Pipe nebula, with the Green Bank Telescope. A statistical equilibrium analysis is applied to the spectra,and the kinetic temperature of a diffuse molecular gas surrounding dense cores is determined accurately; the derived temperature ranges from 40 K to 75 K. From this result, we assess the heating effect on the filamentarystructure of the nebulas stem region due to UV photons from a nearby star $theta$-Ophiuchi and a possible filament-filament collision in the interface of the stem and bowl regions. In the stem region, the gas kinetic temperature is found to be almost independent of the apparent distance from $theta$-Ophiuchi: the UV-heating effect by the star is not visible. On the other hand, the gas kinetic temperature is raised, as high as $sim$75 K, at the interface of the two filamentary structures. This result provides us with an additional support to the filament-filament collision scenario in the Pipe nebula.
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