Speed reduction device for power seat

A speed reduction device for a power seat adapted to be fixed to a seat frame, the speed reduction device comprising:

• a worm gear coaxially and integrally rotating with an output shaft of a motor:
• a helical gear meshed with the worm gear and coaxially and integrally rotating with a drive shaft driving a movable portion of the power seat;
• and
• a casing accommodating the worm gear and the helical gear to be rotatable;
• wherein the casing includes:
• a casing body rotatably supporting the worm gear and including a circular-shaped opening portion through which the helical gear is inserted;
• a casing cover tightened to the casing body and closing the opening portion of the casing body;
• annular fitting portions formed at contact surfaces of the casing body and the casing cover respectively and fitted to each other;
• a first engagement portion formed at an outer circumferential side of the annular fitting portion of one of the casing body and the casing cover;
• a second engagement portion formed at the other of the casing body and the casing cover and engaging with the first engagement portion;
• and
• a tightening means tightening the casing cover to the casing body.