Speed reduction mechanism

A speed reduction mechanism, comprising:

• a rotating shaft;
• a first gear provided on an outer peripheral portion of the rotating shaft and comprising a first tooth portion that extends spirally along a longitudinal direction of the rotating shaft;
• a plurality of second gears each comprising a spiral second tooth portion that meshes with the first tooth portion and arranged side by side in a circumferential direction of the rotating shaft;
• a housing portion housing the plurality of second gears;
• and
• a third gear provided on an inner peripheral portion of the housing portion and comprising a spiral third tooth portion that meshes with the second tooth portion,
• wherein the first tooth portion has at least a part of a tooth profile cross section formed in an arc shape, and
• the second tooth portion meshes with the first tooth portion in a state of being in contact with a portion where the tooth profile cross section is arc-shaped, wherein in a case where a pitch circle radius of the second gear is R1,
• a center-to-center distance from a rotation center of the rotating shaft to a rotation center of the second gear is L1, and
• an angle calculated by a formula of (180°(sum of interior angles of triangle)−90°)−(360°/number of the second gears/2) is θ,
• the pitch circle radius R1<(the center-to-center distance L1×cos θ).