PURPOSE: To facilitate and quicken the opening movement of a cover by forming a space portion at the portion of a covering portion opposite to the end face portion of the cover and supporting the cover, the space portion being compressed by the end face portion of the cover when an air bag is unfolded.

CONSTITUTION: When an air bag 37 is unfolded a rear cover piece 55 presses an upper plate portion 52 downward by means of a protruding streak 47a provided protrusively from the corner portion 47 of a rear end portion 32c and a recessed portion 48 is compressed and in that state the corner portion 47 of the rear end portion 32c of the rear cover piece 55 is deformed at its lower part and is rotated in the direction of a rear opening. Then the opening movement of the rear cover piece 55 is facilitated and quickened so that adjustment of the unfolding performance of the air bag 37 is facilitated. Also, a protruding streak 52a provided protrusively from the end portion of the upper plate portion 52 located above the front end portion 33a of a covering portion 33 either makes contact with the lower part of the protruding streak 47a provided protrusively from the corner portion 47a or faces the streak 47a with a small interval, so that the appearance of the air bag is improved.