A time-dependent analytical solution is found for the velocity of a plane ionization wave generated under nanosecond laser pulse action on the surface of a flat layer of low-Z porous substance with density less than the critical density of the produced plasma. With corrections for the two-dimensional nature of the problem when a laser beam of finite radius interacts with a flat target, this solution is in quantitative agreement with measurements of ionization wave velocity in various experiments. The solution compared with experimental data covering wide ranges of performance conditions, namely, (3–8) × 10¹⁴ W cm^-2 for laser pulse intensity, 0.3–3 ns for pulse duration, 0.35–0.53 μm for laser wavelength, 100–1000 μm for laser beam radius, 380–950 μm for layer thickness, 4.5–12 mg cm^-3 for average density of porous substance, and 1–25 μm for average pore size. The parameters of the laser beam that ensure the generation of a plane ionization wave in a layer of subcritical porous matter are determined for the problem statements and are found to meet the requirements of practical applications.