PURPOSE: To provide a device for tightening a cap of can provided with a nozzle for gas replacement having a simple construction wherein tightening speed of the cap of can is high and air in a head space of the can be surely replaced with gas to improve replacement efficiency.

CONSTITUTION: A nozzle 34 of a gas replacing means 5 is provided in an area extending from a location where a cap Y of a can transferred along an arcuate line by a can cap transfer means 3 comes to above a can vessel X transferred by the first can transfer means 1 to a location where the cap Y is engaged with the vessel X transferred to the second can transfer means 2 moving along a line similar to that of the means 3. A tightening means 6 is provided near the downstream side of the nozzle 34. While the vessel X is being transferred by the means 1 and 2, the cap is applied to the vessel, during which replacement gas is continuously supplied, immediately thereafter the cap Y is tightened by a tightening means 6.