The head-protecting airbag device (M1) includes an airbag (12) provided with a gas admissive portion (13) and a non-admissive portion (23). The gas admissive portion (13) includes a gas feed passage (14) for feeding inflation gas G toward both front and rearwards of the vehicle, and a plurality of vertical chambers (17) juxtaposed along front-rear direction of the vehicle. Each of the vertical chambers (17) includes a communication port at its upper end for communicating with the gas feed passage (14). The substantial lengths of a vehicle's outer wall (13b) and an inner wall (13a) of the airbag (12) are differentiated from each other by a belt (31) in the periphery (19) of the communication port (18). Thus the periphery (19) of the communication port (18) guides inflation gas (G) obliquely down and toward the vehicle's exterior (O) when inflation gas (G) flows into the vertical chamber (17) via the communication port. The head-protecting airbag device (M1) is made compact and improves workability in the mounting work on the vehicle while ensuring downward deployment of the airbag (12) along the window (W).