A steering system that is telescopically extendable and retractable using a motor driving force, the steering system comprising: a motor; a telescopic steering shaft; a supporting member that supports the steering shaft such that the steering shaft is rotatable, the supporting member being movable with telescopic movement of the steering shaft; and a driving force transmission device that transmits a rotational driving force of the motor to the supporting member, wherein the driving force transmission device includes a driving force converter that converts the rotational driving force of the motor to a linear driving force, and a coupling member that couples the driving force converter to one of the motor and the supporting member, the coupling member is breakable by an impact force applied to the coupling member through the steering shaft and the supporting member during a secondary collision so as to decouple the driving force converter from the one of the motor and the supporting member, the driving force converter includes a first member rotatable by the rotational driving force of the motor, and a second member coupled to the supporting member, and the second member engages with the first member in a manner that converts rotational movement of the first member to linear movement of the second member, the coupling member is a part of the second member, characterized in that the second member has a surface that faces an outer circumferential surface of the supporting member, wherein said surface of the second member has contact with the outer circumferential surface of the supporting member and is curved to follow the outer circumferential surface of the supporting member, a tubular recess is recessed from said surface in a direction away from the outer circumferential surface, the recess being located between an internally threaded hole of the second member and said surface, with the coupling member being inserted and fitted in the recess, and a tip portion of the coupling member is inserted and fitted in an engagement hole formed in the outer circumferential surface of the supporting member. a motor; a telescopic steering shaft; a supporting member that supports the steering shaft such that the steering shaft is rotatable, the supporting member being movable with telescopic movement of the steering shaft; and a driving force transmission device that transmits a rotational driving force of the motor to the supporting member, wherein the driving force transmission device includes a driving force converter that converts the rotational driving force of the motor to a linear driving force, and a coupling member that couples the driving force converter to one of the motor and the supporting member, the coupling member is breakable by an impact force applied to the coupling member through the steering shaft and the supporting member during a secondary collision so as to decouple the driving force converter from the one of the motor and the supporting member, the driving force converter includes a first member rotatable by the rotational driving force of the motor, and a second member coupled to the supporting member, and the second member engages with the first member in a manner that converts rotational movement of the first member to linear movement of the second member, the coupling member is a part of the second member, characterized in that the second member has a surface that faces an outer circumferential surface of the supporting member, wherein said surface of the second member has contact with the outer circumferential surface of the supporting member and is curved to follow the outer circumferential surface of the supporting member, a tubular recess is recessed from said surface in a direction away from the outer circumferential surface, the recess being located between an internally threaded hole of the second member and said surface, with the coupling member being inserted and fitted in the recess, and a tip portion of the coupling member is inserted and fitted in an engagement hole formed in the outer circumferential surface of the supporting member.