There is provided a pneumatic bias tire capable of preventing a separation failure between carcass plies in the bending deformation of a tire side portion under an action of a high load. In a heavy duty pneumatic bias tire having a ply rating of not less than 58 and an aspect ratio of not more than 80%, a rubber volume as the product of a distance between ply cords in the carcass ply and a distance between ply cords of mutually adjoining carcass plies with respect to three or more inside carcass plies of an innermost carcass ply group in at least a portion corresponding to a position of a maximum tire width is 500-1350 mm3.