A non-destructive measurement unit of gas concentration in sealed containers and an automatic filling and/or packaging line using such a unit are provided. The flexible containers are at least partially optically transparent, and the measurement unit comprises a light source for emitting a light beam at a wavelength tunable with an absorption wavelength of a gas contained in the sealed flexible container. The light source directs the light beam toward at least one inspection area, and a detector detects at least a portion of the beam after the beam passes through the inspection area and outputs data representative of an absorption spectrum of the gas. Means for generating a head space of predefined width into the sealed flexible container is adapted to advance the sealed flexible container by an advancement path which crosses the inspection zone and to maintain the predefined width of the head space during the advancement.