PURPOSE: To avoid excessive action and improper operation of a pretensioner by improving supply of operating gas from a gas generator to a rotary actuator.

CONSTITUTION: A preparation chamber 102 is formed adjacently to a cylinder chamber 101 in a cylinder block 10 of a rotary actuator 1, and this chamber 102 serves as preparatory release space for impeding directly supplying an impact peak pressure of gas, dynamic pressure and flame in the beginning of operating a gas generator 2, to the cylinder chamber 101. Supply of gas to the cylinder chamber 101 is performed by waiting for a rise of gas pressure in the preparation chamber 102, through a through hole 121 of connecting both these chambers. Thus by preventing an impact excessive load to a related mechanism part (connecting/disconnecting mechanism 3) due to applying an excessive high pressure to the cylinder chamber 101 in the beginning of operating the gas generator 2, gas leakage from the actuator 1 is decreased.