A telescopic steering shaft comprising: a male shaft having an outer circumference on which a plurality of protruding teeth are formed; and a female shaft having an inner circumference on which a plurality of tooth grooves are formed, wherein the tooth grooves of the female shaft are fitted onto the protruding teeth of the male shaft such that the male shaft and the female shaft are relatively slidable in an axial direction and such that a rotational torque is transmittable between the male shaft and the female shaft, wherein a covering portion, made of an elastic material, covers a tooth surface of the protruding teeth to reduce sliding resistance between the protruding teeth and the tooth grooves of the female shaft, wherein a tooth surface of a center part of the covering portion is formed in an arc shape convex toward an outer side of the male shaft in a radial direction, and a curvature radius corresponding to the tooth surface of the center part of the covering portion is smaller than a curvature radius corresponding to a tooth surface of a center part of the tooth grooves, wherein both sides of each engagement region, where the tooth surface of the covering portion and the tooth surface of the tooth grooves engage with each other in a tooth depth direction, are formed with wedge-shaped gaps (62, 63) which widen from a center part of the engagement region toward the both sides in the tooth depth direction, wherein a round chamfered portion, being convex toward an outer side of the female shaft in a radial direction, is provided at a tooth top side of the tooth surface of the covering portion in the tooth depth direction, and a round chamfered portion, being convex toward the outer side of the female shaft in the radial direction, is provided at a tooth root side of the tooth surface of the covering portion in the tooth depth direction, and wherein the tooth surface of the center part of the covering portion, the round chamfered portion of the tooth top side and the round chamfered portion of the tooth root side are convex toward a same direction. a male shaft having an outer circumference on which a plurality of protruding teeth are formed; and a female shaft having an inner circumference on which a plurality of tooth grooves are formed, wherein the tooth grooves of the female shaft are fitted onto the protruding teeth of the male shaft such that the male shaft and the female shaft are relatively slidable in an axial direction and such that a rotational torque is transmittable between the male shaft and the female shaft, wherein a covering portion, made of an elastic material, covers a tooth surface of the protruding teeth to reduce sliding resistance between the protruding teeth and the tooth grooves of the female shaft, wherein a tooth surface of a center part of the covering portion is formed in an arc shape convex toward an outer side of the male shaft in a radial direction, and a curvature radius corresponding to the tooth surface of the center part of the covering portion is smaller than a curvature radius corresponding to a tooth surface of a center part of the tooth grooves, wherein both sides of each engagement region, where the tooth surface of the covering portion and the tooth surface of the tooth grooves engage with each other in a tooth depth direction, are formed with wedge-shaped gaps (62, 63) which widen from a center part of the engagement region toward the both sides in the tooth depth direction, wherein a round chamfered portion, being convex toward an outer side of the female shaft in a radial direction, is provided at a tooth top side of the tooth surface of the covering portion in the tooth depth direction, and a round chamfered portion, being convex toward the outer side of the female shaft in the radial direction, is provided at a tooth root side of the tooth surface of the covering portion in the tooth depth direction, and wherein the tooth surface of the center part of the covering portion, the round chamfered portion of the tooth top side and the round chamfered portion of the tooth root side are convex toward a same direction.