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; a casing (74, 135, 76) including a casing body (74, 135) rotatably supporting the worm gear and including a circular-shaped opening portion through which the helical gear is inserted, the casing (74, 135, 76) including a casing cover tightened to the casing body (74, 135) and closing the opening portion of the casing body (74, 135), the casing (74, 135, 76) being fixed to a housing (68, 136) of the motor; and a motor fixation device including; a first fitting portion (64, 141, 137, 138) provided at the housing (68, 136) and surrounding the output shaft of the motor, a first engagement portion (64a, 136b) formed at the first fitting portion (64, 141) in a direction perpendicular to the output shaft of the motor, a second fitting portion (74a, 135a, 110) formed at the casing body (74, 135) and to which the first fitting portion (64, 138) is fitted, a second engagement portion (74b, 135b) formed at the second fitting portion (74a, 110) in a direction perpendicular to the output shaft of the motor and arranged to overlap the first engagement portion in a direction perpendicular to the output shaft of the motor in a case where the first fitting portion (64, 138) is fitted to the second fitting portion (74a, 135a), and an engagement projection formed at the casing cover and engaging with the first engagement portion (64a, 136b) and the second engagement portion (74b, 135b) in a case where the casing cover is tightened to the casing body (74, 135), characterized in that the second fitting portion formed at the casing body includes a fitting hole while the first fitting portion provided at the housing includes a fitting protrusion fitted to the fitting hole in a manner that the fitting protrusion is prevented from rotating relative to the fitting hole, and a cylindrical fitting portion is formed at an end of the fitting protrusion in a projecting manner and in a coaxial manner with the output shaft of the motor, the cylindrical fitting portion being fitted to a cylindrical fitting bore that is formed at the casing body in a coaxial manner with the output shaft of the motor at an end portion of the fitting hole away from the motor. 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; a casing (74, 135, 76) including a casing body (74, 135) rotatably supporting the worm gear and including a circular-shaped opening portion through which the helical gear is inserted, the casing (74, 135, 76) including a casing cover tightened to the casing body (74, 135) and closing the opening portion of the casing body (74, 135), the casing (74, 135, 76) being fixed to a housing (68, 136) of the motor; and a motor fixation device including; a first fitting portion (64, 141, 137, 138) provided at the housing (68, 136) and surrounding the output shaft of the motor, a first engagement portion (64a, 136b) formed at the first fitting portion (64, 141) in a direction perpendicular to the output shaft of the motor, a second fitting portion (74a, 135a, 110) formed at the casing body (74, 135) and to which the first fitting portion (64, 138) is fitted, a second engagement portion (74b, 135b) formed at the second fitting portion (74a, 110) in a direction perpendicular to the output shaft of the motor and arranged to overlap the first engagement portion in a direction perpendicular to the output shaft of the motor in a case where the first fitting portion (64, 138) is fitted to the second fitting portion (74a, 135a), and an engagement projection formed at the casing cover and engaging with the first engagement portion (64a, 136b) and the second engagement portion (74b, 135b) in a case where the casing cover is tightened to the casing body (74, 135), a first fitting portion (64, 141, 137, 138) provided at the housing (68, 136) and surrounding the output shaft of the motor, a first engagement portion (64a, 136b) formed at the first fitting portion (64, 141) in a direction perpendicular to the output shaft of the motor, a second fitting portion (74a, 135a, 110) formed at the casing body (74, 135) and to which the first fitting portion (64, 138) is fitted, a second engagement portion (74b, 135b) formed at the second fitting portion (74a, 110) in a direction perpendicular to the output shaft of the motor and arranged to overlap the first engagement portion in a direction perpendicular to the output shaft of the motor in a case where the first fitting portion (64, 138) is fitted to the second fitting portion (74a, 135a), and an engagement projection formed at the casing cover and engaging with the first engagement portion (64a, 136b) and the second engagement portion (74b, 135b) in a case where the casing cover is tightened to the casing body (74, 135),