Sodium ion and ammonium ion were adsorbed by the ion exchange method on granular porous vycor glass at 298 K. An equation to estimate the amount of ion exchanged was introduced, assuming that solid acid contains mutiple kinds of ion exchange sites which have different acidities is and ion exchange equilibria respectively. A numerical analysis is carried out in order that the estimated values are fitted in with the experimental values for the amount of ion exchanged. It reveals two kinds of ion exchange equilibria, and give the apparent ion exchange equilibrium constants and effective amounts of acid. Then,the amounts of sodium ion and ammonium ion exchanged can be estimated. The apparent ion exchange equilibrium constants of sodium ion corresponding to the acidity index, that is, pKa of 6.41 and 8.72, suggest that porous glass is a considerablely weak dibasic solid acid. It is considered that such acidity of porous glass is due to geminal-type and vicinal-type of silanol groups and that isolate types do not contribute to ion exchange.