PURPOSE: To restrict emission of striking sounds of a valve, etc., to the outside by surrounding at least the body part of an air-fuel injection device with a sound insulating cover, and ventilating the inside of the sound insulating cover by air in the downstream of the air-fuel injection device on the injection compressed air path.

CONSTITUTION: An air-fuel injection device 19 loaded on the upper part of each combustion chamber 17 at the upper face of a cylinder head 4, is composed of an injection body 20 inserted and fixed to the cylinder head 4, injection ports of a fuel chamber and a fuel chamber formed in the injection body 20 and an opening and closing valve mechanism 21, and a fuel injection valve 22 for supplying fuel through the injection body 20 into the combustion chamber 17. In front of each injection device 19, an air rail 40 is provided so that it covers an air-introducing port 24d. In this case, a box-like sound insulating cover 64 is provided so that it surrounds the injection body 20 and the air rail 40, and fixed with bolts 66a, 66b. The inside of the sound insulating cover 64 is ventilated by air in the downstream of the injection device on the compressed air path.