The line strength ratios of lines having a common upper rovibronic level but belonging to different P, Q, and R branches (rotational branching ratios) are, for the first time, measured for the diagonal (v′ = v″ = 0-3) bands of the I¹Π_g^- , J¹Δ_g^- → C¹Π_u^± electronic transitions of the D₂ molecule. Within the framework of the nonadiabatic model, the ratio of the dipole moments of the 3d¹Δ_g → 2p¹Π_uand 3d¹Π_g → 2p¹Π_u electronic transitions. M_2pπ^3dΔ/M_2pπ^3dπ, was obtained from the previously calculated expansion coefficients in the Born- Oppenheimer basis set of perturbed wave functions of the I¹Π_g^- and J¹Δ_g^- states of the D₂ molecule and experimental values of rotational branching ratios. It was found that, as in the case of H₂, there is a significant discrepancy between the values of M_2pπ^3dΔ/M_2pπ^3dπ derived from the analysis of experimental data on different electronic transitions: I¹Π_g^- → C¹Π_u^± and J¹Δ_g^- → C¹Π_u^±. The reasons for this discrepancy are discussed. At the same time, when the data on both molecules are considered on an equal footing, the effective value of M_2pπ^3dΔ/M_2pπ^3dπ = -1.42 ± 0.01, which agrees well with the value of -1.35 ± 0.05 that we obtained earlier for transitions between similar triplet states of H₂ and D₂ and is close to the estimate of - √2 in the pure precession approximation, corresponds to the whole body of the measured rotational branching ratios of the molecules H₂ and D₂.