A knee protecting air bag apparatus M1 is mounted to a column cover 16 for covering a column cover 16 for covering a steering column. In operating the apparatus M1, a contained air bag 45 is supplied with an expanding gas G and is projected from the column cover 16 to be able to protect the knee of a driver while being expanded to develop. The column cover 16 includes a door portion 34 opened by being pressed by the air bag to be able to form a projecting opening 22 of the air bag 45 at two left ad right side faces 18b, 18c from a side of a lower face 18a. Inside of the air bag 45 is arranged with a gas flow restricting member 64 such that the expanding gas G is made to flow to a left door portion 36 and a right door portion 37 of the door portion 34 by being directed in outer directions of both left and right sides.