A pedestrian protection airbag device adapted to be mounted on a vehicle, the airbag device including a housing and an airbag (60, 60A, 60B) which is stored in the housing in a folded-up configuration and inflatable with an inflation gas and deployable towards the rear, left and right via a clearance formed between a rear end of a hood and a cowl of the vehicle for covering the cowl and left and right front pillars (5, 5L, 5R) of the vehicle; the airbag (60, 60A, 60B) includes a horizontal inflatable portion that is deployable in such a manner as to extend in a left and right direction to cover an upper surface of the cowl and left and right vertical inflatable portions (72, 72A, 72B, 72L, 72R) that are deployable in such a manner as to extend rearward from vicinities of left and right ends of the horizontal inflatable portion to cover upper surfaces of the front pillars (5, 5L, 5R); a circumferential wall of the airbag (60, 60A, 60B) is composed of a vehicle body facing wall deployable on a lower side and a pedestrian-facing wall deployable on an upper side, the vehicle body facing wall and pedestrian-facing wall being continuous at outer peripheral edges thereof; the horizontal inflatable portion includes: an in-hood inflatable portion that includes an inlet port (68, 68L, 68R) for letting in an inflation gas and is deployable beneath the rear end of the hood; and an out-of-hood inflatable portion that is deployable in such a manner as to protrude rearward out of the rear end of the hood and is in gas communication with the vertical inflatable portions (72L, 72R) at vicinities of left and right ends (70c, 70d) of the out-of-hood inflatable portion; the airbag (60, 60A, 60B) further includes a band-shaped partitioning tether (74, 74A, 74B) that is jointed to the vehicle body facing wall and pedestrian-facing wall by upper and lower edges of the partitioning tether (74, 74A, 74B) and partitions an inflatable region of the airbag (60, 60A, 60B) as fully inflated ranging from the out-of-hood inflatable portion to the left and right vertical inflatable portions (72L, 72R) into a rear end region and a front region, the rear end region being shaped along a rear edge of the airbag; the front region and an inflatable region of the airbag disposed in front of the front region serve as a primary inflatable region and the rear end region serves as a secondary inflatable region that admits the inflation gas from the primary inflatable region and starts inflation after the primary inflatable region starts inflation, characterized in that; the partitioning tether (74, 74A, 74B) is so disposed as to form supply ports (80, 80L, 80R) via which the secondary inflatable region admits the inflation gas from the primary inflatable region only at vicinities of left and right ends (74a, 74b) of the partitioning tether (74, 74A, 74B). an in-hood inflatable portion that includes an inlet port (68, 68L, 68R) for letting in an inflation gas and is deployable beneath the rear end of the hood; and an out-of-hood inflatable portion that is deployable in such a manner as to protrude rearward out of the rear end of the hood and is in gas communication with the vertical inflatable portions (72L, 72R) at vicinities of left and right ends (70c, 70d) of the out-of-hood inflatable portion;