PURPOSE: To obtain an engine casing which is simply constructed with a small number of parts, by forming storm rails, projecting upward from both right and left sides of said casing, as integrated members.

CONSTITUTION: Storm rails 19, projecting upward from both right and left sides of an engine casing 15, are formed as integrated members with the interior as a hollow and upper faces as sloping surfaces 23 ascending rearward. A ventilation opening 25 is fitted to the sloping surface 23 and air flowing in from this opening is led into an engine room through a passage built below the storm rail. Further, a hatch 21 leading to the engine room is fitted to a concave space 20 formed by the storm rail 19.