A uniflow scavenging two-cycle engine comprising: a cylinder in which a combustion chamber is formed; a piston that slides in the cylinder; a scavenging port that is provided one end side of the piston in a stroke direction in the cylinder and suctions active gas in the combustion chamber in response to a sliding motion of the piston; and a fuel injecting unit that is provided on an outer side of the cylinder in a radial direction from the scavenging port, characterized in that the fuel injecting unit injects fuel gas into the active gas which is suctioned into the scavenging port, characterized in that the fuel injecting unit includes: an inner pipe which has an inner hole penetrating between an inside and an outside thereof and through which fuel gas is guided to the inside, an outer pipe which has an outer hole penetrating between an inside and an outside thereof and stows the inner pipe in the inside thereof so as to form a double pipe with the inner pipe, and a driving unit which changes relative positions of the inner pipe and the outer pipe in the stroke direction of the piston and changes an opening amount as an area of an overlap between the inner hole and the outer hole, wherein an opening region formed of the overlap between the inner hole and the outer hole includes a small-flow-quantity opening region (Os) that is positioned relatively on one end side in the stroke direction of the piston and a large-flow-quantity opening region (Ob) that is positioned on the other end side in the stroke direction of the piston from the small-flow-quantity opening region (Os). a cylinder in which a combustion chamber is formed; a piston that slides in the cylinder; a scavenging port that is provided one end side of the piston in a stroke direction in the cylinder and suctions active gas in the combustion chamber in response to a sliding motion of the piston; and a fuel injecting unit that is provided on an outer side of the cylinder in a radial direction from the scavenging port, characterized in that the fuel injecting unit injects fuel gas into the active gas which is suctioned into the scavenging port, characterized in that the fuel injecting unit includes: an inner pipe which has an inner hole penetrating between an inside and an outside thereof and through which fuel gas is guided to the inside, an outer pipe which has an outer hole penetrating between an inside and an outside thereof and stows the inner pipe in the inside thereof so as to form a double pipe with the inner pipe, and a driving unit which changes relative positions of the inner pipe and the outer pipe in the stroke direction of the piston and changes an opening amount as an area of an overlap between the inner hole and the outer hole, wherein an opening region formed of the overlap between the inner hole and the outer hole includes a small-flow-quantity opening region (Os) that is positioned relatively on one end side in the stroke direction of the piston and a large-flow-quantity opening region (Ob) that is positioned on the other end side in the stroke direction of the piston from the small-flow-quantity opening region (Os). an inner pipe which has an inner hole penetrating between an inside and an outside thereof and through which fuel gas is guided to the inside, an outer pipe which has an outer hole penetrating between an inside and an outside thereof and stows the inner pipe in the inside thereof so as to form a double pipe with the inner pipe, and a driving unit which changes relative positions of the inner pipe and the outer pipe in the stroke direction of the piston and changes an opening amount as an area of an overlap between the inner hole and the outer hole, wherein an opening region formed of the overlap between the inner hole and the outer hole includes a small-flow-quantity opening region (Os) that is positioned relatively on one end side in the stroke direction of the piston and a large-flow-quantity opening region (Ob) that is positioned on the other end side in the stroke direction of the piston from the small-flow-quantity opening region (Os).