Provided is a method of efficiently treating a fluid to be treated containing a compound that destroys a zeolite membrane to prevent the fluid from destroying the zeolite membrane. A fluid to be treated 10 formed of a liquid mixture or a gas mixture and containing a compound that destroys a zeolite membrane 2 is brought into contact with particles (3, 5) made of the same type of zeolites as the zeolite membrane 2 and filling a pretreatment device 4 installed upstream of a membrane module 1 including the zeolite membrane 2 or a portion upstream of the zeolite membrane 2 in the membrane module 1 to destroy the zeolite forming the particles (3, 5) and the fluid to be treated 10 is made to contain a component generated by the destruction.