A steering assistance device (100; 100A; 100B; 100C), comprising: a motor (200; 200A; 200B; 200C) configured to output a drive force in accordance with a steering force applied to a steering shaft (102; 102A); and a speed reducer (300; 300A; 300B; 300C) including: a gear section configured to rotate and oscillate in accordance with the drive force; an outer cylinder having an internally toothed ring formed thereon, the internally toothed ring being configured to mesh with the gear section; and a carrier configured to rotate in accordance with rotation and oscillation of the gear section and output a steering assistance force to a steering mechanism (STM; STN), the steering mechanism (STM; STN) being configured to steer vehicle wheels (LTR; RTR) in accordance with rotational operation applied to a steering wheel (101; 101A), wherein an engagement ratio between the gear section and the internally toothed ring is from 25 to 100% characterized in that the speed reducer (300; 300A; 300B; 300C) includes: an input gear configured to receive the drive force from the motor (200; 200A; 200B; 200C); and a crank mechanism (820; 840) configured to transmit the drive force received by the input gear to the gear section, so as to produce the rotation and oscillation, the input gear reduces a speed of the rotational motion transmitted from the motor (200; 200A; 200B; 200C) at a first reduction ratio, and the gear section reduces the speed of the rotational motion transmitted from the input gear at a second reduction ratio larger than the first reduction ratio. a motor (200; 200A; 200B; 200C) configured to output a drive force in accordance with a steering force applied to a steering shaft (102; 102A); and a speed reducer (300; 300A; 300B; 300C) including: a gear section configured to rotate and oscillate in accordance with the drive force; an outer cylinder having an internally toothed ring formed thereon, the internally toothed ring being configured to mesh with the gear section; and a carrier configured to rotate in accordance with rotation and oscillation of the gear section and output a steering assistance force to a steering mechanism (STM; STN), the steering mechanism (STM; STN) being configured to steer vehicle wheels (LTR; RTR) in accordance with rotational operation applied to a steering wheel (101; 101A), a gear section configured to rotate and oscillate in accordance with the drive force; an outer cylinder having an internally toothed ring formed thereon, the internally toothed ring being configured to mesh with the gear section; and a carrier configured to rotate in accordance with rotation and oscillation of the gear section and output a steering assistance force to a steering mechanism (STM; STN), the steering mechanism (STM; STN) being configured to steer vehicle wheels (LTR; RTR) in accordance with rotational operation applied to a steering wheel (101; 101A), wherein an engagement ratio between the gear section and the internally toothed ring is from 25 to 100% characterized in that the speed reducer (300; 300A; 300B; 300C) includes: an input gear configured to receive the drive force from the motor (200; 200A; 200B; 200C); and a crank mechanism (820; 840) configured to transmit the drive force received by the input gear to the gear section, so as to produce the rotation and oscillation, an input gear configured to receive the drive force from the motor (200; 200A; 200B; 200C); and a crank mechanism (820; 840) configured to transmit the drive force received by the input gear to the gear section, so as to produce the rotation and oscillation, the input gear reduces a speed of the rotational motion transmitted from the motor (200; 200A; 200B; 200C) at a first reduction ratio, and the gear section reduces the speed of the rotational motion transmitted from the input gear at a second reduction ratio larger than the first reduction ratio.