PURPOSE: To make the maintenance and check easy, by providing an accommodation chamber surrounded by a cover on a lid plate located at the upper portion of a tank, and accommodating a compressor and engine accessories in the accommodation chamber.

CONSTITUTION: An engine body 4 is mounted on the lower side of a lid plate 3 covering an upper portion of a tank 1 under a submerged condition. An accommodation chamber 21 is formed between a cover 20 and the lid plate 3 to accommodate a compressor 16 and engine accessories such as an air cleaner 22, a regulator 28 and a mixer 25. Accordingly, the engine accessories may be checked or repaired at a stroke by removing the single cover.