A steering system comprising: an electric motor; a wheel turning shaft that is moved in an axial direction by a torque of the electric motor so as to turn a wheel-to-be-turned; a plurality of transmission devices; a rotation angle detection unit that detects a rotation angle of the electric motor; and a controller that controls the electric motor, wherein: one of the transmission devices is a first transmission device that outputs rotation of the electric motor at a reduced speed; characterized in that the controller has an angular speed detection unit (45, 52) that calculates an angular speed of the first transmission device based on the rotation angle and a combined friction torque estimation unit that estimates a combined friction torque combining friction torques occurring in the respective transmission devices; and the combined friction torque estimation unit has a slipping speed calculation part that calculates a slipping speed of the first transmission device based on the angular speed, a friction coefficient calculation part that calculates a friction coefficient of the first transmission device based on the slipping speed, a tooth flank normal force calculation part (84, 86) that calculates a normal force acting on a tooth flank of the first transmission device, and a friction torque calculation part (83, 85, 87, 88, 89, 90, 91) that calculates the combined friction torque using the friction coefficient of the first transmission device, the tooth flank normal force of the first transmission device, and one or more preset correction factors. an electric motor; a wheel turning shaft that is moved in an axial direction by a torque of the electric motor so as to turn a wheel-to-be-turned; a plurality of transmission devices; a rotation angle detection unit that detects a rotation angle of the electric motor; and a controller that controls the electric motor, wherein: one of the transmission devices is a first transmission device that outputs rotation of the electric motor at a reduced speed; characterized in that the controller has an angular speed detection unit (45, 52) that calculates an angular speed of the first transmission device based on the rotation angle and a combined friction torque estimation unit that estimates a combined friction torque combining friction torques occurring in the respective transmission devices; and the combined friction torque estimation unit has a slipping speed calculation part that calculates a slipping speed of the first transmission device based on the angular speed, a friction coefficient calculation part that calculates a friction coefficient of the first transmission device based on the slipping speed, a tooth flank normal force calculation part (84, 86) that calculates a normal force acting on a tooth flank of the first transmission device, and a friction torque calculation part (83, 85, 87, 88, 89, 90, 91) that calculates the combined friction torque using the friction coefficient of the first transmission device, the tooth flank normal force of the first transmission device, and one or more preset correction factors. one of the transmission devices is a first transmission device that outputs rotation of the electric motor at a reduced speed; characterized in that the controller has an angular speed detection unit (45, 52) that calculates an angular speed of the first transmission device based on the rotation angle and a combined friction torque estimation unit that estimates a combined friction torque combining friction torques occurring in the respective transmission devices; and the combined friction torque estimation unit has a slipping speed calculation part that calculates a slipping speed of the first transmission device based on the angular speed, a friction coefficient calculation part that calculates a friction coefficient of the first transmission device based on the slipping speed, a tooth flank normal force calculation part (84, 86) that calculates a normal force acting on a tooth flank of the first transmission device, and a friction torque calculation part (83, 85, 87, 88, 89, 90, 91) that calculates the combined friction torque using the friction coefficient of the first transmission device, the tooth flank normal force of the first transmission device, and one or more preset correction factors.