To provide a head part protection air bag and its device capable of achieving gentle pressure fluctuation in each chamber and facilitating pressure adjustment in a first chamber.

When an automobile collides on its side face or falls on its side, a gas generator operates to supply gas into the first chamber (cushion chambers 21 to 29) between a ventilating panel 10 and a first panel 11 of the air bag 1 in order to inflate the air bag 1. This air bag 1 spreads downward like a curtain and inflates between an occupant and a side face of a vehicle compartment in the automobile. When the inflated cushion chambers 21 to 29 receive occupant's head part, gas permeates through the permeable panel 10 and flows into a second chamber 58 when internal pressure in the cushion chambers 21 to 29 exceeds predetermined pressure.