A speed change device has an input member rotatable in opposite directions to transmit higher torque as its rotation angle becomes larger, and a variable angle-ratio change mechanism including rollers and teethed wheels that are arranged in parallel to each other. The rollers and teethed wheels are respectively connected to rotate together with each other. The teeth of the wheels have a clearance on a pitch circle between their adjacent teeth so that the clearance is larger than a slip amount of the rollers in a low torque range where the rollers transmit low torque with bypassing the wheels in a normal operation. The wheels are engaged with each other to transmit high torque in a high torque range where the high torque is applied in an exceptional operation and the rollers slip larger than the slip amount in the low torque range.