To provide a storing case for a magnetic disk cartridge in which a circular plate-like magnetic disk is rotatably stored in a flat cartridge shell having each of through-pass holes in at least two corners of the four corners in it and further prevent a stored magnetic disk cartridge from being influenced by an excessive shrinking stress even if resin having a high thermal shrinkage rate as compared with that of resin used in the cartridge shell is used in the storing case.

A storing case 60 is provided with steps 65 supporting a cartridge shell at least three locations on its surface in a thickness direction. Supporting columns 66 inserted into each of through-pass holes 26, 28 of the cartridge shell are protruded and arranged on at least a pair of surfaces of these steps 65 and at the same time one of a pair of supporting columns 66, 66 is fitted to the through-pass hole 26 to support the cartridge shell in a planar direction, and the remaining supporting column 66 is loosely fitted to the through-pass hole 28.