A linear actuator, comprising: a main body of a hollow structure having an open end portion and a closed end portion; a cylindrical cylinder (12e, 23) is fixed to a closed end portion side in the main body, and has an open end at an open end portion side; a linear motion rod having one end portion extending from the open end portion of the main body, and the other end portion having a wear ring on an outer circumferential portion, the wear ring being housed in the cylindrical cylinder (12e, 23) slidably in an axial direction, and the linear motion rod having a slide section that is slidable in the axial direction via a linear motion oil seal of the open end portion on an outer periphery of the open end portion side, the linear motion oil seal and the sliding section sealing an interior of the main body; and a rotary hollow shaft supported in the main body rotatably about an axis of the linear motion rod, wherein the rotary hollow shaft has a permanent magnet on a part of an outer circumferential surface, the main body has a stator winding section at a position which faces the permanent magnet, on an inner surface thereof, and the permanent magnet and the stator winding section configure a hollow motor, the rotary hollow shaft has a nut section that is fixed coaxially, the linear motion rod has a screw section which engages with the nut section, on an outer circumferential surface between the slide section and the wear ring, and the nut section and the screw section are engaged with each other to configure a rotation/linear motion converting mechanism in which by the rotary hollow shaft being rotated, the linear motion rod linearly moves in the axial direction, in both sides in the axial direction of the linear motion rod, of the nut section, two rotary oil seal sections (15, 16, 18, 19) which seal gaps between the rotary hollow shaft and members fixed to the main body are included, and the rotary oil seal sections (15, 16, 18, 19) define a first sealed region which is a space in the main body in which the linear motion rod linearly moves and in which a lubricating oil and a gas coexist, by separating the first sealed region from a second sealed region, in which the hollow motor (11, 20) is disposed, and seal the first sealed region in cooperation with the linear motion oil seal section. a main body of a hollow structure having an open end portion and a closed end portion; a cylindrical cylinder (12e, 23) is fixed to a closed end portion side in the main body, and has an open end at an open end portion side; a linear motion rod having one end portion extending from the open end portion of the main body, and the other end portion having a wear ring on an outer circumferential portion, the wear ring being housed in the cylindrical cylinder (12e, 23) slidably in an axial direction, and the linear motion rod having a slide section that is slidable in the axial direction via a linear motion oil seal of the open end portion on an outer periphery of the open end portion side, the linear motion oil seal and the sliding section sealing an interior of the main body; and a rotary hollow shaft supported in the main body rotatably about an axis of the linear motion rod, wherein the rotary hollow shaft has a permanent magnet on a part of an outer circumferential surface, the main body has a stator winding section at a position which faces the permanent magnet, on an inner surface thereof, and the permanent magnet and the stator winding section configure a hollow motor, the rotary hollow shaft has a nut section that is fixed coaxially, the linear motion rod has a screw section which engages with the nut section, on an outer circumferential surface between the slide section and the wear ring, and the nut section and the screw section are engaged with each other to configure a rotation/linear motion converting mechanism in which by the rotary hollow shaft being rotated, the linear motion rod linearly moves in the axial direction, in both sides in the axial direction of the linear motion rod, of the nut section, two rotary oil seal sections (15, 16, 18, 19) which seal gaps between the rotary hollow shaft and members fixed to the main body are included, and the rotary oil seal sections (15, 16, 18, 19) define a first sealed region which is a space in the main body in which the linear motion rod linearly moves and in which a lubricating oil and a gas coexist, by separating the first sealed region from a second sealed region, in which the hollow motor (11, 20) is disposed, and seal the first sealed region in cooperation with the linear motion oil seal section.