A head-protecting airbag according to the present invention is normally folded and housed in an upper edge of windows inside a vehicle, and deployable downward from the upper edge of windows upon inflow of inflation gas to cover the vehicle's inner side of the windows. The airbag internally includes a tether joined to the vehicle's inner side wall and the vehicle's outer side wall for regulating the clearance between the inner side wall and the outer side wall as the airbag is completely inflated. The tether is disposed along the front-rear direction of the vehicle. A joint portion of the tether to the vehicle's inner side wall is located at lower level than a joint portion of the tether to the vehicle's outer side wall as viewed in a vertical section taken along the vehicle's transverse direction. The head-protecting airbag of the present invention is able to expand along the windows.