To allow a stable stacking without spoiling the design and even with different type of cases.

A portable type musical instrument storing case is provided with a strip-shaped recess for stacking in a longitudinal direction of one surface of either a base plate or a cover plate and a strip-shaped protrusion for stacking on the other surface that is engaged with the concave, both formed by a single mould. The strip-shaped recess and the strip-shaped protrusion are molded with predetermined width and depth or height and positioned in a longitudinal direction of shells of different size and/or shape to allow stacking with different type of cases.