An apparatus for testing the tightness of containers (1) closed by respective membranes (2) comprises a deforming unit (15) configured to exert a deforming action on a plurality of said containers (1), and a detection unit (40) for detecting the deformation of the membrane (2) of each container (1) submitted to the deforming action, wherein said deforming unit (15) comprises a plurality of deforming devices (20), and each deforming device (20) is individually associated with a single and respective container (1) of said plurality of containers to exert said deforming action.