An airbag apparatus having a gas generator assembly for a restraining device, comprising: a gas generator including, a housing accommodating a gas generating member, a cup-shaped diffuser having a gas discharge port for discharging gas generated from the gas generating member and being attached to one end of the housing, a cup-shaped gas flow controller assembled with the gas generator, an outer diameter of the diffuser being smaller than an outer diameter of the housing, a stud bolt being provided to protrude from a bottom surface of the diffuser, the gas flow controller being in a cup-like shape having a bottom surface, a circumferential wall portion and an opening portion, the bottom surface being provided with a hole for receiving the stud bolt of the diffuser therethrough, the stud bolt of the diffuser being put through the hole of the gas flow controller from the opening portion, the circumferential wall portion of the gas flow controller surrounding the gas discharge port of the diffuser, the circumferential wall portion of the gas flow controller and a circumferential wall portion of the diffuser being spaced therebetween, the gas flow controller being attached such as to form a gap between a peripheral edge of the opening portion and the housing, characterized in that the airbag apparatus further comprises a module case and an airbag accommodated by the module case, the gas generator assembly is attached to an inside of the module case accommodating the airbag, the bottom surface of the gas flow controller is provided with a communication hole for allowing passage of gas flow, the hole for allowing passage of the stud bolt therethrough is formed at a central part of the bottom surface, and the plurality of communication holes are formed radially to surround the hole, and gas, discharged from the gas discharge port of the gas generator, colliding the circumferential wall portion of the gas flow controller, the gas flow being divided into both sides in the axial direction to flow out from the gap and the communication hole. a gas generator including, a housing accommodating a gas generating member, a cup-shaped diffuser having a gas discharge port for discharging gas generated from the gas generating member and being attached to one end of the housing, a cup-shaped gas flow controller assembled with the gas generator, an outer diameter of the diffuser being smaller than an outer diameter of the housing, a stud bolt being provided to protrude from a bottom surface of the diffuser, the gas flow controller being in a cup-like shape having a bottom surface, a circumferential wall portion and an opening portion, the bottom surface being provided with a hole for receiving the stud bolt of the diffuser therethrough, the stud bolt of the diffuser being put through the hole of the gas flow controller from the opening portion, the circumferential wall portion of the gas flow controller surrounding the gas discharge port of the diffuser, the circumferential wall portion of the gas flow controller and a circumferential wall portion of the diffuser being spaced therebetween, the gas flow controller being attached such as to form a gap between a peripheral edge of the opening portion and the housing, the airbag apparatus further comprises a module case and an airbag accommodated by the module case, the gas generator assembly is attached to an inside of the module case accommodating the airbag, the bottom surface of the gas flow controller is provided with a communication hole for allowing passage of gas flow, the hole for allowing passage of the stud bolt therethrough is formed at a central part of the bottom surface, and the plurality of communication holes are formed radially to surround the hole, and gas, discharged from the gas discharge port of the gas generator, colliding the circumferential wall portion of the gas flow controller, the gas flow being divided into both sides in the axial direction to flow out from the gap and the communication hole.