The solution reservoir apparatus of the capillary electrophoresis apparatus securely affixes an evaporation-preventing membrane to a container when a capillary is inserted or withdrawn, without extending the cathode end of the capillary. The solution reservoir apparatus comprises a container for reserving a sample or solution, a cover having a bore through which the capillary is passed and covering the container, an evaporation-preventing membrane having a capillary hole through which the capillary is passed, and a container holder for holding the container. The evaporation-preventing membrane has a projection provided at the periphery of the capillary hole, the projection of the evaporation-preventing membrane is engaged with the bore on the cover when the evaporation-preventing membrane is positioned on the cover, and the evaporation-preventing membrane is supported by the cover.