A pedestrian protection airbag device adapted to be mounted on a backside of and in a vicinity of a rear end of a hood of a vehicle, the airbag device comprising: an airbag to be stored at the backside of the hood in a folded-up configuration; an inflator for feeding an inflation gas to the airbag; and a case disposed on the backside of the hood; the airbag device further includes a mounting bracket (56, 56A, 56B) that supports the in flator and mounts the airbag and the inflator on the hood; wherein the mounting bracket includes: an inflator holding section (59, 59A, 59B) that holds the inflator; and a mounting section (58, 58L, 58R, 58F, 58A) that is adapted to be secured to the hood; wherein the mounting bracket covers an underside of the airbag and includes a guide plate (63, 63C) that helps deploy the airbag rearwardly as the airbag inflates with the in flation gas. wherein the airbag is stored in the case in a folded-up configuration and inflatable with an inflation gas for rearward deployment; the airbag is stored at the rear of the inflator in the case; and wherein the mounting bracket (56, 56A, 56B) supports the inflator and mounts the inflator on the hood, characterized in that: the case is composed of an upper case and a lower case so as to cover the airbag and the inflator entirely, wherein the upper case includes a ceiling wall which covers the airbag in a folded-up configuration and the inflator from above, a generally square tubular circumferential wall extending downward from the peripheral edge of the ceiling wall and a flange which extends outwardly in four directions from the lower end of the circumferential wall, wherein the lower case includes a bottom wall which covers the undersides of the airbag and inflator, an inner wall extending upward from the peripheral edge of the bottom wall, and a flange which extends outwardly in four directions in such a manner as to continue from the bottom wall; wherein, when the inflator is actuated and the airbag inflates with an inflation gas, the airbag pushes and deforms a rear wall of the circumferential wall of the upper case and a rear region of the inner wall of the lower case, and the rear wall and inner wall open rearward and form a gap, thus allowing the airbag to deploy rearward and upward. an airbag to be stored at the backside of the hood in a folded-up configuration; an inflator for feeding an inflation gas to the airbag; and a case disposed on the backside of the hood; the airbag device further includes a mounting bracket (56, 56A, 56B) that supports the in flator and mounts the airbag and the inflator on the hood; wherein the mounting bracket includes: an inflator holding section (59, 59A, 59B) that holds the inflator; and a mounting section (58, 58L, 58R, 58F, 58A) that is adapted to be secured to the hood; wherein the mounting bracket covers an underside of the airbag and includes a guide plate (63, 63C) that helps deploy the airbag rearwardly as the airbag inflates with the in flation gas. wherein the airbag is stored in the case in a folded-up configuration and inflatable with an inflation gas for rearward deployment; the airbag is stored at the rear of the inflator in the case; and wherein the mounting bracket (56, 56A, 56B) supports the inflator and mounts the inflator on the hood, characterized in that: the case is composed of an upper case and a lower case so as to cover the airbag and the inflator entirely, wherein the upper case includes a ceiling wall which covers the airbag in a folded-up configuration and the inflator from above, a generally square tubular circumferential wall extending downward from the peripheral edge of the ceiling wall and a flange which extends outwardly in four directions from the lower end of the circumferential wall, wherein the lower case includes a bottom wall which covers the undersides of the airbag and inflator, an inner wall extending upward from the peripheral edge of the bottom wall, and a flange which extends outwardly in four directions in such a manner as to continue from the bottom wall; wherein, when the inflator is actuated and the airbag inflates with an inflation gas, the airbag pushes and deforms a rear wall of the circumferential wall of the upper case and a rear region of the inner wall of the lower case, and the rear wall and inner wall open rearward and form a gap, thus allowing the airbag to deploy rearward and upward. the case is composed of an upper case and a lower case so as to cover the airbag and the inflator entirely, wherein the upper case includes a ceiling wall which covers the airbag in a folded-up configuration and the inflator from above, a generally square tubular circumferential wall extending downward from the peripheral edge of the ceiling wall and a flange which extends outwardly in four directions from the lower end of the circumferential wall, wherein the lower case includes a bottom wall which covers the undersides of the airbag and inflator, an inner wall extending upward from the peripheral edge of the bottom wall, and a flange which extends outwardly in four directions in such a manner as to continue from the bottom wall; wherein, when the inflator is actuated and the airbag inflates with an inflation gas, the airbag pushes and deforms a rear wall of the circumferential wall of the upper case and a rear region of the inner wall of the lower case, and the rear wall and inner wall open rearward and form a gap, thus allowing the airbag to deploy rearward and upward.