A speed reduction mechanism and a motor equipped with the speed reduction mechanism are provided. A pinion gear is provided with one spiral engagement projected part, and a helical gear is provided with a plurality of engagement recessed parts with which the engagement projected part is engaged. The engagement projected part and the engagement recessed part are formed so that their cross-sectional shapes along a direction orthogonal to an axial direction of the pinion gear are in arc shapes. A shape of helical teeth and a shape of the engagement recessed part are determined based on a shape of the engagement projected part provided on a spiral tooth.