To accurately detect a fine leakage of empty cans in a short time by mounting an empty can inner pressure detector to an empty can in an inlet station, and a pressure change of a gas fed into the empty can is detected for a conveyance period during which the detector is detached therefrom in an outlet station.

A mounter 7 is lowered and an empty can inner pressure detector 5 is mounted to an empty can 1 in an inlet station 3, and also an inner pressure detecting air is fed to the empty can 1. The empty can 1 is conveyed on a conveying track 2 keeping the detector 5, and when reaching an outlet station 4, the detector 5 is detached from the empty can 5 by a detacher 8, and is returned to a return track 6. The detector 5 detects an inner pressure of the empty can 1 during the conveyance period therefor, and supplies it to a detection part provided in the mounter 7 or the detacher 8, and the detection part measures an inner pressure dropping amount at start and end times of pressure change measurement in the empty can 1. While the empty cans 1 are sequentially conveyed, a transition of can inner pressure change is observed and a leakage of the empty can 1 is detected from the inclination.