Shaft retention mechanism and speed reducer

A speed reducer comprising:

• a casing forming a housing space containing a lubricant;
• a speed reducing mechanism contained in the housing space;
• a shaft penetrating a through-hole formed in the casing, the shaft extending through the casing, the shaft being connected to the speed reducing mechanism in the casing;
• a sealing member encircling the shaft in the through-hole, the sealing member sealing between the shaft and the casing;
• and
• a bearing disposed on an opposite side of the sealing member to the housing space, the bearing rotatably supporting the shaft in the through-hole,
• wherein the sealing member includes a first annular portion contacting an inner peripheral surface of the through-hole and extending in an axial direction, a second annular portion extending inward in a radial direction from the first annular portion, and a third annular portion connected to the second annular portion and contacting an outer peripheral surface of the shaft,
• wherein the inner peripheral surface of the through-hole is provided with a step surface that an end on an opposite side of the first annular portion to the bearing contacts,
• wherein a width of the step surface in the radial direction is less than a thickness of the first annular portion in the radial direction,
• wherein the shaft includes an outer shaft rotatably supported by the bearing, and an inner shaft, which are assembled together,
• wherein the inner shaft includes a fixed portion fixed in the outer shaft, and a projecting portion positioned on the housing space side of the fixed portion and in which an outer peripheral surface thereof is in contact with the third annular portion of the sealing portion, and
• wherein an outer diameter of the projecting portion is less than that of a portion of the outer shaft, the portion of the outer shaft being supported by the bearing.