To improve working environment for using a pneumatically acting device by letting no air be exhausted out of an opening close to an operator.

A gripping part 12 is provided for the lower part of a device main body 1, jaws 4 for letting a C-ring be deformed is provided for the device main body 1, an air cylinder 3 for rotating the jaws 4 is provided for the device main body 1, a direction change-over valve 5 connected with the air cylinder 3 is provided for the device main body 1, and an exhaust air flow path 10 communicated with the exhaust port 5b of the direction change-over valve 5 is provided for the device main body 1. And the exhaust air flow path 10 is opened to the lower part of the gripping part 12, an exhaust air flow path 11 comminicated with the exhaust port 5b of the direction change-over valve 5 is provided for the device main body 1, the exhaust air flow path 11 is opened to the rear side of the device main body 1, and an air hose 21 is also provided, which connects the opening part 11a of the exhaust air flow path 11 with an original part 10a of the exhaust air flow path 10.