To provide a method of manufacturing a porous glass in which the porosity decreases as a function of the distance from the surface in the direction of depth. A method of manufacturing a porous glass includes a step of bringing one or more than one ion species selected from silver ion, potassium ion and lithium ion into contact with a matrix glass containing borosilicate glass as main ingredient and heating the matrix glass to form a glass body having an ion concentration distribution with a concentration of the one or more than one ion species decreasing as a function of a distance from a surface in a direction of depth, a step of heating and phase-separating the glass body to form a phase-separated glass, and a step of etching the phase-separated glass to form a porous glass having a porosity decreasing as the function of the distance from the surface in the direction of depth.