An airbag cover has lid portions which are opened by a breakable portion being broken when the airbag cover is pushed by an airbag which is being inflated and is formed from a synthetic resin by molding into a single layered airbag cover. A breakable portion on a distal side of the lid portions is formed as a distal edge straight section through cutting operation which utilizes a cutting blade. The part is formed by providing a thick portion and a thin portion so that a maximum change rate which is a rate of difference between a maximum value and a minimum value based on the maximum value is made to be 15% or more without providing at both ends of a sectional area change curve marked by plotting along the part values of sectional areas of a segment of 50 mm along the part an area where the values of the sectional areas are reduced from a center towards both ends thereof as a range where the values of the sectional areas are larger than the minimum value.