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.