PURPOSE: To provide a decompression device of an engine where the structure is simple, the production cost is low, the working is smooth, and the structure is miniaturized and can be fitted to a cam shaft without increasing the size of the cam shaft.

CONSTITUTION: In an engine having a cam shaft 5 where a gear 1 for driving a cam shaft or a sprocket and a cam 4 to control the opening/closing of an intake valve and an exhaust valve through a tuppet 3 are coaxially arranged, a moving body which is provided with an arm 8a abutted on the tuppet 3 and can be moved in the axial direction on the cam shaft, a return spring 9 to provide the energizing force in the axial direction with the moving body, and a ball 6 which is formed between the gear and the moving body, stored in the clearance which is reduced toward the outer circumference, and can be rolled in the radial direction are provided.