PURPOSE: To make easy and inexpensive in production of a mechanism for preventing a seat belt and an airbag from being unintentionally operated at the time of handling or assembling.

CONSTITUTION: A mechanical sensor 26 of a restraining means 18 for a safety belt restraint system is guaranteed to prevent unintentional release in non- assembly state by engaging a pin 42 with an inertia mass 30 of a high vehicle sensitivity and restrains the inertia mass 30 at an operation preparation completing position thereof. The pin 42 is connected through a coated cable 24 to a component 52 held in a support housing 50 and attached so as to be rotated. A rotatable member 52 has an annular disk shape and therefore, an attaching screw 16 can be inserted through a hole 62 at the center thereof. When fastening the attaching screw 16, the component 52 is also rotated together. Therefore, the pin 42 is pulled through an extension cable 24b and the restraint of the inertia mass 30 is canceled.