Disclosed is a pharmaceutical composition comprising at least one excipient selected from a sugar and a sugar alcohol each having an equilibrium moisture content of 1% by weight or lower at 25˚C and 70% relative humidity and oseltamivir phosphate, wherein the sugar or the sugar alcohol contains glucose and mannose as impurities each in an amount of 0.01% by weight or less relative to the weight of the sugar or the sugar alcohol.