A worm gear mechanism includes a worm wheel having a central axis; and a worm shaft made of resin having a rotational axis and a plurality of geared portions which engages with the worm wheel, the worm shaft allowing the worm wheel to rotate about the central axis of the worm wheel when rotating about the rotational axis of the worm shaft, tooth root portions of the geared portions being configured to have a profile which varies gradually along an axial direction of the worm shaft relative to a datum line being perpendicular to the axial direction of the worm shaft and passing through the central axis of the worm wheel.