A pyrotechnic gas generator intended for motor-vehicle safety includes a combustion chamber of cylindrical or approximately cylindrical shape. An initiator is located within the combustion chamber and is associated with at least one solid pyrotechnic charge. A diffusion chamber communicates with the combustion chamber and receives gases generated by the pyrotechnic charge and transfers the gases to atmosphere via at least one evacuation opening. The diffusion chamber extends parallel to the longitudinal axis of the combustion chamber, in the extension of one of its end faces. The diffusion chamber is also of generally cylindrical or approximately cylindrical shape, but with a cross section that is less than that of the combustion chamber. The diffusion chamber is devoid of structure for filtering the gases that pass therethrough.