[en] The nuclear spin-lattice relaxation was studied in Bi₄Ge₃O₁₂ single crystal by ²⁰⁹Bi NQR. Experimental recovery curves for all ²⁰⁹Bi NQR transitions of nuclear spin I=((9)/(2)) in the temperature range 10-230 K were described by the single effective spin-lattice relaxation time T₁^*. The temperature dependence of 1/T₁^* was close to Tⁿ law with n=2.4-2.7. Theoretical treatment was given for the nuclear spin-lattice relaxation in the multi-level system for the case of single-axial crystalline electric field. Both quadrupole and magnetic mechanisms of relaxation were taken into account. It was shown that magnetic mechanism contributes noticeably to the spin-lattice relaxation in Bi₄Ge₃O₁₂ at T≤50 K