We report on the 29Si nuclear spin decoherence time at room temperature for a pure (99.99999%, 7N) and carrier-less (p-type, doping level of $10^{15}$ cm-3) silicon in which 29Si nuclei are naturally abundant (4.7%). Despite the experimental difficulties coming from the extremely long spin-lattice relaxation time $T_{1}$ (of the order of $10^{4}$ s), we have successfully observed a series of spin-echoes of which the time dependence is characterized by the decoherence time $T_{2}$. We found that the decoherence process deviates from a single Gaussian or Lorentzian function but is well-reproduced by a bi-exponential function with the shorter ${T_{2 } }^{\text{S } }=15{\pm} 5$ ms and the longer ${T_{2 } }^{\text{L } }=200{\pm} 20$ ms.