A packaging machine for producing closed packages (21) includes a chamber (17) that can be sealed in an airtight manner around one or a plurality of packages, a gas flushing unit (34) for flushing with a gas the at least one package (21) contained in the chamber (17), at least one gas line (32, 33) leading into the chamber (17) or out of the chamber (17) as well as a measuring device (35) for determining a concentration of the gas. The measuring device (35) comprises a measuring head (37) for reading, by means of electromagnetic radiation, an indicator (39) for the concentration of the gas. The indicator is arranged in an interior of the chamber (17), in an interior of a package (21) and/or in an interior the at least one gas line (32, 33).