Abstract: In order to construct a rail supporting means on the casing for a power driven chain saw in such a way that, when the saw chain is placed in position, the saw rail is already retained relatively firmly and is adjustable by means of the saw chain tightening device, a casing for a power driven chain saw is provided with a drive unit driving the saw chain revolvingly via the saw rail with a rail supporting means with two fastening bolts penetrating a slot-like aperture constructed in the saw rail and a saw chain tightening device arranged within the area of the rail supporting means and a sprocket wheel cover which can be secured to the casing that overlaps and covers the rail supporting means, the sprocket wheel cover (10) being constructed in two portions and comprising a protective cover (20) and a clamping cover (30), the clamping cover (30) being provided with two perforations (12a, 13a) through which the bolts (12,13) are able to penetrate and it being possible for said clamping cover to be mounted by mean