A head protecting airbag is folded up to be housed at an upper part of a window of a vehicle so as to be deployed and inflated downward from the upper part of the window so as to cover an interior side of the window. In the airbag, a plurality of parallel cells adapted to be deployed and inflated by an inflating gas introduced thereinto are provided in parallel with each other below a gas supply passage portion in which the inflating gas flows along a longitudinal direction. In a cell of the parallel cells so provided which is required to deploy quickly, of front and rear partitioning portions, a partitioning portion situated at an upstream side of the gas flowing in the gas supply passage portion is provided such that an end thereof which extends upward from a lower edge portion is disposed so as to form an inlet port between a distal end of a horizontal portion of a downstream side partitioning portion and itself.