To investigate the air permeability of sugi (Cryptomeria japonica) and the effect of grain directions on it, the air permeabilities of air-dried sugi sapwood and heartwood were determined along the three material directions of wood. The value of the longitudinal permeability was the highest and that of the radial permeability was the lowest. The permeability of heartwood was about an order of magnitude less than the permeability of sapwood in the same direction. The ratio between the tangential and radial permeability was approximately 10, which was similar to softwoods that have impermeable rays. These results suggest that sugi has ray tissues that are either impermeable or have very low permeability. The radial permeability of sugi was much lower than that of Pinus, Sequoia, Juniperus, Abies and Tsuga measured with gases reported in the literature, indicating that sugi is one of the least radially permeable softwoods. These findings explain the reasons for the difficulties encountered in the drying and chemical treatment of sugi (Cryptomeria japonica).