PURPOSE: To make the built-in volume at minimum and insure assembling accuracy and assembling facility of a safe operation part by providing a positioning means between both, in addition to an air bag module fitting part on the fixedly securing face between a housing having an operation mechanism of the lock member of a safety device and an inflator.

CONSTITUTION: A housing 5 in which a part of constituting members of a safety device is built in and modularly coordinated is outward inserted from below to an inflator 4. In this case, in addition to bolt inserting holes 4c, 5b formed on the respective four corners of the respective flanges 4a, 5a of the inflator 4 and the housing 5, positioning pins 17 are projected at the two corner parts of the flange 4a of the inflator 4, and positioning holes 18 are provided on the corresponding positions of the flange 5a of the housing 5. This working is accurately performed because malfunction of a starting device is caused due to even a small dislocation. Meanwhile. the diameter of the bolt inserting holes 4c, 5b is provided with a surplus so as to insert the bolts smoothly.