Abstract: A membrane heat treatment method includes a process of raising the temperature of a membrane to an intermediate heating temperature (step S21), a process of heating and keeping the membrane at the intermediate heating temperature (step S22), a process of raising the temperature of the membrane to a main heating temperature higher than the intermediate heating temperature (step S23), and the process of heating and keeping the membrane at the main heating temperature (step S24). A first recovery amount R1 that is a difference in permeability of the membrane between after step S22 and before step S21 is 50% or more and 95% or less of a second recovery amount R2 that is a difference in permeability of the membrane between after step S24 and before step S21.