PURPOSE: To enhance the charging effect of intake-air with a structure which is simple and excellent in durability, by disposing a reed valve between a crank chamber and an intake-air passage opened to the crank chamber, and by providing a thin plate-like rotary valve which is rotatably integral with a crankshaft and slidable in the axial direction.

CONSTITUTION: When a crankshaft 4 is rotated so that a piston 2 ascends, a negative pressure is effected in a crank chamber 3 to open a reed valve 14, thereby fuel gas from an intake-air passage 15 flows into the crank chamber 3. Further, a rotary valve 19 rotated in association with the crankshaft 4 opens an intake-air passage 17 at a certain rotational angle due to the provision of a notch 22, and fuel gas branched off from the intake-air passage 15 into a second intake-air passage 16 is fed into the crank chamber 3. Meanwhile, when the piston 2 reaches a position near to the top dead center, an intake-air port 17 is closed, and further, when the piston 17 exceeds the top dead center, a positive pressure is effected in the crank chamber 3 to close the reed valve 14. With this arrangement, it is possible to enhance the charging efficiency of intake-air, and to provided an intake-air device which is simple in its structure and which has a large durability.