The measurements of the magnetic susceptibility, electron-spin-resonance (ESR) and heat capacity on the title substance have revealed that the substance is a typical S=1/2 Heisenberg antiferromagnetic chain. The intrachain exchange interaction energy is estimated to be J/k=-55K. No three-dimensional long range ordering due to interchain interaction has been observed down to 1.1K. Furthermore, the critical behavior of the magnetic susceptibility at low temperatures, which has been predicted theoretically by Eggert et al. ( Phys. Rev. Lett. 73 (1994) 332), is successfully examined. The ESR linewidth of a single crystal, which strongly depends on the direction of the external magnetic filed. is well explained on the basis of the spin-diffusion theory for one-dimensional magnets.