The vibrational branching ratios (the ratios of the line strengths of Q branches) are measured for the first time in the v″ progressions of bands of the I¹ Π_g ^-, v′, N → B¹ Σ_u ⁺, v″ transitions in the H₂, molecule for v′ = 0-3. v″ = 0-8, and N = 1-4. A significant discrepancy (reaching a factor of five) is found between the experimental data and the results of calculation in the Franck-Condon approximation. From a semiempirical analysis of our experimental data, the dependence of the dipole moment of the 3d¹Π_g → 2p¹Σ_u ⁺ electronic transition on the internuclear separation r is obtained. It is found that, for r = 1.5-3.0 au, a linear approximation of this dependence allows an adequate description of the whole set of the measured ratios of the transition probabilities. The r dependence of the dipole moment predicted by the semiempirical analysis is approximately twice as strong as that derived from the results of semiempirical calculations available in the literature. Relative values of the dependences of the (I¹ Π_g ^-, J¹ Δ_u ^- ). v′, N → B¹ Σ_u ⁺, v″ radiative transition probabilities on the vibrational quantum numbers v′ and v″ and the rotational quantum number N are obtained for v′ = 0-3. v″ = 0-9, and N = 1-6.