An oil-free screw compressor comprising: a screw rotor including a screw and a shaft; a bearing supporting the shaft; a first shaft seal device fitted on the shaft, the first shaft seal device disposed between the screw and the bearing, the first shaft seal device including: a first seal opposite to the shaft, and a first communication section configured to communicate a portion of an inner peripheral face of the first shaft seal device on the bearing side with respect to the first seal, and an outer peripheral face of the first shaft seal device; a second shaft seal device fitted on the shaft, the second shaft seal device disposed between the first shaft seal device and the bearing, the second shaft seal device including: a second seal opposite to the shaft, and a second communication section configured to communicate a portion of an inner peripheral face of the second shaft seal device on the screw side with respect to the second seal, and an outer peripheral face of the second shaft seal device; a casing (12, 112, 212, 312, 412, 612, 712) including a rotor chamber accommodating the screw, and a shaft accommodation space accommodating the shaft, the bearing, the first shaft seal device, and the second shaft seal device; and a first seal member (50, 954) fitted on the shaft, the first seal member (50, 954) including an annular seal configured to be opposite to the shaft between the first communication section and the second communication section and separate the first communication section and the second communication section, wherein the casing (12, 112, 212, 312, 412, 612, 712) includes an atmosphere communication section (12m, 112m, 212m, 312m, 412m, 612m, 712m) connected to both of the first communication section and the second communication section on the inner peripheral face of the shaft accommodation space, the atmosphere communication section (12m, 112m, 212m, 312m, 412m, 612m, 712m) configured to communicate the first communication section and the second communication section with an atmosphere, and wherein a flow passage sectional area in the first communication section is larger than a flow passage sectional area between the seal of the first seal member (50, 954) and the shaft, characterized in that an angular position of the first communication section with respect to a rotation center of the shaft is different from an angular position of the second communication section. a screw rotor including a screw and a shaft; a bearing supporting the shaft; a first shaft seal device fitted on the shaft, the first shaft seal device disposed between the screw and the bearing, the first shaft seal device including: a first seal opposite to the shaft, and a first communication section configured to communicate a portion of an inner peripheral face of the first shaft seal device on the bearing side with respect to the first seal, and an outer peripheral face of the first shaft seal device; a first seal opposite to the shaft, and a first communication section configured to communicate a portion of an inner peripheral face of the first shaft seal device on the bearing side with respect to the first seal, and an outer peripheral face of the first shaft seal device; a second shaft seal device fitted on the shaft, the second shaft seal device disposed between the first shaft seal device and the bearing, the second shaft seal device including: a second seal opposite to the shaft, and a second communication section configured to communicate a portion of an inner peripheral face of the second shaft seal device on the screw side with respect to the second seal, and an outer peripheral face of the second shaft seal device; a second seal opposite to the shaft, and a second communication section configured to communicate a portion of an inner peripheral face of the second shaft seal device on the screw side with respect to the second seal, and an outer peripheral face of the second shaft seal device; a casing (12, 112, 212, 312, 412, 612, 712) including a rotor chamber accommodating the screw, and a shaft accommodation space accommodating the shaft, the bearing, the first shaft seal device, and the second shaft seal device; and a first seal member (50, 954) fitted on the shaft, the first seal member (50, 954) including an annular seal configured to be opposite to the shaft between the first communication section and the second communication section and separate the first communication section and the second communication section, wherein the casing (12, 112, 212, 312, 412, 612, 712) includes an atmosphere communication section (12m, 112m, 212m, 312m, 412m, 612m, 712m) connected to both of the first communication section and the second communication section on the inner peripheral face of the shaft accommodation space, the atmosphere communication section (12m, 112m, 212m, 312m, 412m, 612m, 712m) configured to communicate the first communication section and the second communication section with an atmosphere, and wherein a flow passage sectional area in the first communication section is larger than a flow passage sectional area between the seal of the first seal member (50, 954) and the shaft, characterized in that an angular position of the first communication section with respect to a rotation center of the shaft is different from an angular position of the second communication section.