For geological disposal of radioactive waste and large-scale collapse of rock slopes due to heavy rain fall, it is significantly important to determine the water transport in the rock mass including unsaturated zones. To precisely predict the two-phase flow of air and water in rock pore spaces, both the hydraulic conductivity and gas permeability must be accurately identified. Most existing methods estimate the air permeability from the differential pressure under steady-state conditions. There are still few methods for evaluating air permeability under unsteady state conditions. This study proposes a method of identifying the ever-changing permeability with time and pressure using a linearized governing equation for air permeability in porous media. This method is based on an exact solution of the linear diffusion equation and requires that air pressure be measured at three different coordinates to approximate the second-order partial derivative with respect to the coordinates. To validate the method, we compared the exact solution derived with the results of air permeability laboratory tests on granite and conglomerate in previous studies. After confirming that the exact solution well represented the results of previous experiments, the unsteady air permeability of the rock was estimated from the results of the same experiments using the proposed method. As a result, it was confirmed that not only can the air permeability be identified with the same accuracy as the steady-state method, but also the unsteady air permeability, which varies with pressure, can be identified.