A membrane distillation method comprising use of a membrane distillation apparatus comprising: an evaporation part comprising a liquid phase part 1 in which water to be treated flows, a gas phase part 1, and a hydrophobic porous membrane that separates the liquid phase part 1 and the gas phase part 1, a condensation part comprising a liquid phase part 2 in which cooling water flows, a gas phase part 2, and a cooling member that separates the liquid phase part 2 and the gas phase part 2, and a gas phase part 3 connecting the gas phase part 1 with the gas phase part 2, wherein the hydrophobic porous membrane is a hydrophobic porous hollow fiber membrane wherein the inner surface of the hydrophobic porous membrane forms the surface that comes into contact with the water to be treated, wherein the surface porosity of the inner membrane side of the hydrophobic porous hollow fiber membrane is 20% or more and 35% or less, wherein the air permeation coefficient of the hydrophobic porous membrane is 8.0 × 10 -7 m 3 /m 2 ·sec·Pa or more and 1.0 × 10 -5 m 3 /m 2 ·sec·Pa or less, wherein the hydrophobic porous membrane comprises at least one resin selected from the group consisting of polysulfone, polyethersulfone, polyethylene, polypropylene, polyvinylidene fluoride, and polytetrafluoroethylene, wherein the surface porosity is measured by analyzing the image of an electron micrograph as described herein, and wherein the air permeation coefficient is determined according to the method described herein, wherein the pressure of the gas phase parts 1 to 3 is 1 kPa or higher and lower than or equal to a pressure lower by at least 5 kPa than the saturated steam pressure of water at the temperature of water to be treated. an evaporation part comprising a liquid phase part 1 in which water to be treated flows, a gas phase part 1, and a hydrophobic porous membrane that separates the liquid phase part 1 and the gas phase part 1, a condensation part comprising a liquid phase part 2 in which cooling water flows, a gas phase part 2, and a cooling member that separates the liquid phase part 2 and the gas phase part 2, and a gas phase part 3 connecting the gas phase part 1 with the gas phase part 2, wherein the hydrophobic porous membrane is a hydrophobic porous hollow fiber membrane wherein the inner surface of the hydrophobic porous membrane forms the surface that comes into contact with the water to be treated, wherein the surface porosity of the inner membrane side of the hydrophobic porous hollow fiber membrane is 20% or more and 35% or less, wherein the air permeation coefficient of the hydrophobic porous membrane is 8.0 × 10 -7 m 3 /m 2 ·sec·Pa or more and 1.0 × 10 -5 m 3 /m 2 ·sec·Pa or less, wherein the hydrophobic porous membrane comprises at least one resin selected from the group consisting of polysulfone, polyethersulfone, polyethylene, polypropylene, polyvinylidene fluoride, and polytetrafluoroethylene, wherein the surface porosity is measured by analyzing the image of an electron micrograph as described herein, and wherein the air permeation coefficient is determined according to the method described herein, wherein the pressure of the gas phase parts 1 to 3 is 1 kPa or higher and lower than or equal to a pressure lower by at least 5 kPa than the saturated steam pressure of water at the temperature of water to be treated. <EOS>