To provide a speed reduction gear for a power seat capable of reducing a production cost by reducing the number of parts for assembling a motor to the speed reduction device, processing man-hour, and assembling man-hour, and also reliably assembling the motor to the speed reduction device.

A motor fixation device 87 includes a fitting portion 64 provided at a housing 63 surrounding an output shaft of the motor 62, a housing side engaged portion 64a formed at the fitting portion 64 in a direction perpendicular to the output shaft of the motor 62, an engaged portion 74a formed at a casing body 74 to which the fitting portion 64 is fitted, a casing side engaged portion 74b formed at the engaged portion 74a in a direction perpendicular to the output shaft of the motor 62 and aligned with the housing side engaged portion 64a in the case where the fitting portion 64 is fitted into the engaged portion 74a, and an engagement projection 76a formed in a casing cover body 76, engaged with the housing side engaged portion 64a and the casing side engaged portion 74b in the case where the casing cover body 76 is tightened to the casing body.