PURPOSE: An aqueous solution contaning mannitol is subjected to drying where the water evaporation is effected at temperature lower than specific one to evaporate a specific amount of water at the beginning, thus producing the objective freeze-dried product used for an injection solution or the like containing mannitol as a diluent.

CONSTITUTION: In freeze drying of an aqueous solution of mannitol, the evaporation conditions are adjusted not so as to exceed -22°C in vaporization and at least 1%, preferably at least 5% water is vaporized at the beginning step, thus producing the objective freeze-dried product. The concentration of mannitol in the aqueous solution is preferably 1W10% based on the total water.

EFFECT: The freeze (continuous line)-thawing (dotted line) curve in the mannitol aqueous solution has a peak of the amount of unfreezing water at -22°C, which is caused by precipitation of the solute, and so a part of water is removed at a temperature lower than -22°C to reduce the unfreezing water and lower the shock by refreezing.