A pneumatic spinning device that forms a yarn by twisting, by whirling airflow, fibers passing through a spinning chamber, the pneumatic spinning device comprising: a fiber guiding section having a first passage through which a fiber bundle passes; and a hollow guide shaft body that has a second passage through which the fiber bundle having passed through the first passage is guided, and that is provided so as to face the fiber guiding section across the spinning chamber, wherein the first passage is provided so as to extend linearly, the second passage is provided so as to extend along a shaft center of the hollow guide shaft body with the shaft center as a center, a downstream end where the first passage opens into the spinning chamber and an upstream end where the second passage opens into the spinning chamber are arranged with an interval in a direction of the shaft center of the hollow guide shaft body, an inner surface of the first passage has a flat plane portion on a side close to the shaft center of the hollow guide shaft body, an angle of a linear portion obtained by cutting the plane portion with any plane perpendicular to a first direction in which the first passage extends with respect to the first direction is constant between an upstream end and the downstream end of the first passage, the plane portion is arranged to deviate with respect to the shaft center of the hollow guide shaft body in a second direction perpendicular to a direction of the shaft center of the hollow guide shaft body, and characterized in that an interval in the second direction between a downstream end of the plane portion and the shaft center of the hollow guide shaft body is equal to or greater than 0.8 mm and equal to or less than 3.4 mm; wherein a length of the plane portion in the first direction is equal to or greater than 3 mm and equal to or less than 12 mm; wherein a passage area of the first passage when the fiber guiding section is cut with a plane perpendicular to the first direction is equal to or greater than 4 mm 2 and equal to or less than 7 mm 2. a fiber guiding section having a first passage through which a fiber bundle passes; and a hollow guide shaft body that has a second passage through which the fiber bundle having passed through the first passage is guided, and that is provided so as to face the fiber guiding section across the spinning chamber, wherein the first passage is provided so as to extend linearly, the second passage is provided so as to extend along a shaft center of the hollow guide shaft body with the shaft center as a center, a downstream end where the first passage opens into the spinning chamber and an upstream end where the second passage opens into the spinning chamber are arranged with an interval in a direction of the shaft center of the hollow guide shaft body, an inner surface of the first passage has a flat plane portion on a side close to the shaft center of the hollow guide shaft body, an angle of a linear portion obtained by cutting the plane portion with any plane perpendicular to a first direction in which the first passage extends with respect to the first direction is constant between an upstream end and the downstream end of the first passage, the plane portion is arranged to deviate with respect to the shaft center of the hollow guide shaft body in a second direction perpendicular to a direction of the shaft center of the hollow guide shaft body, and characterized in that an interval in the second direction between a downstream end of the plane portion and the shaft center of the hollow guide shaft body is equal to or greater than 0.8 mm and equal to or less than 3.4 mm; wherein a length of the plane portion in the first direction is equal to or greater than 3 mm and equal to or less than 12 mm; wherein a passage area of the first passage when the fiber guiding section is cut with a plane perpendicular to the first direction is equal to or greater than 4 mm 2 and equal to or less than 7 mm 2.