To provide a deceleration driving mechanism using a rotation angle sensor having sufficient detection accuracy, capable of miniaturization and reduction of the number of components.

In this deceleration driving mechanism 20 stored in a gear box 21, and constituted of a driving motor 26, a deceleration gear train comprising gears 27a, 27b, 27c, and the rotation angle sensor 28 comprising a polarized magnetic substance 28a and a magnetometric sensor 28c, a D-shaped through hole 28b is provided in the magnetic substance 28a of the rotation angle sensor 28, and the sectional shape of one end of an output shaft 29 of the final stage gear 27c of the deceleration gear train is chamfered so that the section has the D-shape so as to be engaged with the through hole 28b of the magnetic substance 28a, to thereby acquire a D-shaped engaging shaft part 27d. Consequently, angle phase determination and rotation stop can be performed without ricketiness only by inserting the shaft part 27d into the through hole 28b of the magnetic substance 28a.