A membrane defect inspection method that can detect damage in a filtrate membrane and can detect presence or absence of damage or a seal defect in a membrane module; and the method is for a membrane module set including multiple membrane modules connected under gas detection piping communicating with primary spaces of the multiple membrane modules where raw water is supplied or secondary spaces in multiple membrane modules where treated water is extracted after the raw water is filtrated by membranes. The method includes a gas injection process where gas is injected into spaces opposite the primary or secondary spaces communicating with gas detection piping in the multiple membrane modules while the gas detection piping is filled with water, and a vibration detection process where a vibration sensor is brought into contact with a protrusion protruding outward from the gas detection piping to detect vibration of the gas detection piping.

A watertightness testing method for testing watertightness of a joined section (4) where one end section of a second pipe (3) is joined to a first pipe (2), the watertightness testing method including: inserting a testing device body (21) into the first pipe (2); attaching an assembling device (80) to another end section of the second pipe (3); attaching a pulling device (82a) to the assembling device (80) and connecting the pulling device (82a) to a strap-shaped member (112a) having been wound in advance around an outer circumference of the first pipe (2); operating the pulling device (82a) and pulling the strap-shaped member (112a) in a separating direction of the second pipe (3) to have a reaction force generated in the second pipe (3) cause the second pipe (3) to be pulled in a joining direction (G), the one end section of the second pipe (3) to be inserted into the first pipe (2), and the second pipe (3) to be joined to the first pipe (2); and operating a moving operation rod (22) provided in the testing device body (21) from outside of the other end section of the second pipe (3) in a state where the second pipe (3) is being pulled in the joining direction (G) to move the testing device body (21) to the joined section (4).