an axial direction similarly to the ignition agent housing chamber 16 the combustion chamber 18 may be formed with a space in which the gas generating agent 24 does not exist however when an excessively large space roughly 10 or more of the total volume for example is formed a gas generation amount decreases and therefore the space is preferably made as small as possible fig2 is a graph showing experiment results relating to comparative reference examples and embodiments of the present invention fig2 shows temporal variation in a post ignition tank pressure of the gas generator when a fill ratio of the ignition agent 22 is adjusted relative to the volume of the ignition agent housing chamber 16 the vertical axis shows the tank pressure and the horizontal axis shows the passage of time from the moment at which ignition is performed by the initiator fig2 shows first and second comparative reference examples and first second and third embodiments fig3 is a table showing relative fill amounts of the gas generating agent 24 and the ignition agent 22 in the comparative reference examples and the embodiments of the present invention as shown in fig2 with the structure of this embodiment the tank pressure at an initial activation stage of the gas generator airbag device can be suppressed below that of the comparative reference examples when the fill ratio of the ignition agent is about 50 or less when the fill ratio.