A porous glass having high strength and a low refractive index, an optical member by using the porous glass, and a method for manufacturing the porous glass are provided. A method for manufacturing a porous glass includes the steps of heat-treating a glass body, which can be phase-separated through heating and which is formed from a plurality of components, at a first temperature to effect the phase separation, heat-treating the glass body, which has been heat-treated at the first temperature, at a second temperature, higher than the first temperature, to effect the phase separation, and bringing the glass body, which has been heat-treated at the second temperature, into contact with an aqueous solution, wherein the total time of the heat treatment time at the first temperature and the heat treatment time at the second temperature is 7 hours or more.