A membrane defect inspection method is for a membrane module set including a plurality of membrane modules connected in parallel under a straight pipe portion of gas detection piping extending in a horizontal direction and communicating with primary spaces in the plurality of membrane modules to which raw water is supplied or secondary spaces. The method includes a gas injection process where gas is injected into spaces opposite to the primary spaces or the secondary spaces communicating with the gas detection piping while the gas detection piping is filled with water, and an echo detection process where an ultrasonic sensor is brought into contact with an end portion of the straight pipe portion of the gas detection piping, and a reflected wave of an ultrasonic wave transmitted from the ultrasonic sensor is detected.