A motor with speed reduction mechanism is provided with an anti-scattering cover 80 for grease, which holds grease applied to a mating part MT where a worm 36b and a worm wheel 40 are in engagement with each other, the anti-scattering cover 80 for grease is provided and closer to an opening 32c than the worm wheel 40 along an axial direction of the worm wheel 40. The anti-scattering cover 80 for grease has: a first wall 81 extending from the opening 32c toward a bottom 32a and formed between the worm 36b and a side wall 32b of a speed reduction mechanism housing unit 32; and a second wall 82 extending in a direction intersecting with the first wall 81 and covering the mating part MT from the same side as the opening 32c along the axial direction of the worm wheel 40. The first wall 81 and the second wall 82 of the anti-scattering cover 80 for grease hold grease attached to the worm 36b, thereby preventing exhaustion of grease applied to the mating part MT where worm 36b and the worm wheel 40 are in engagement with each other.