A gas generator comprising a housing (3) made of metal and having an initiator shell (1) and a closure shell (2), (a) combustion chamber(s) (5, 5a, 5b) formed in the housing (3) and packed with gas generant (4) that generates high temperature gas by burning, (a) filter member(s) (6, 6a, 6b) arranged around the combustion chamber(s) (5, 5a, 5b), (an) igniter device(s) (7, 7a, 7b) put in the housing (3) for igniting and burning the gas generant (4) in the combustion chamber(s) (5, 5a, 5b), and a plurality of gas discharge holes (8) formed at the housing (3), for discharging gas generated in the combustion chamber(s) (5, 5a, 5b), wherein either or both of the initiator shell (1) and the closure shell (2) forming the housing (3) has a head portion (14, 10) of a semispherical form or an oval semispherical form and a cylindrical portion (13, 9) of a diameter (D) formed continuously from the head portion (14, 10), and wherein a ratio H/D between the diameter D of the cylindrical portion (13, 9) and a distance H between bottoms of the head portions (14, 10) of the initiator shell (1) and closure shell (2) is in the range of 0.4-1.3.