A flat heavy-duty pneumatic radial tire having a 0-degree belt layer formed of steel cords and increased in durability, wherein multiple plies of steel cord belt layers (6) are disposed on the outer periphery of a carcass layer (4). The steel cord belt layers (6) includes at least one ply of the 0-degree belt layer with a cord angle of substantially 0° relative to the circumferential direction of the tire and at least two plies of bias belt layers (8) with a cord angle substantially equal to an equilibrium angle of within a range of 45° to 65° relative to the circumferential direction of the tire. The tire is manufactured as follows. The tire cured and molded in a mold is released from the mold, assembled with a rim to be inflated while the tire is hot, and then cooled to normal temperature under the inflated condition.