A detachable case assembly is disclosed to include a case body, which has front and rear angled slots, a partition plate, which separates the case body into a front and a rear holding space, a front cover, which covers the front half of the top side of the case body and which has locating rods for engaging into the front angled slots of the case body and a rear edge supported on the protruded bearing portions of the partition plate, and a rear cover, which covers the rear half of the top side of the case body and which has locating rods for engaging into the rear angled slots of the case body, a front flange supported on the partition plate and spaced below the rear edge of the front cover, raised contact portions maintained in close contact with the rear edge of the front cover.