There is provided a chain tension applying device for a bicycle that can accommodate a change in an axial distance between a frame and a sprocket. A chain tensioner 40 is a device for applying tension to a chain 26, including: a bracket 56; a tension member 58; a securing position adjustment mechanism 60; and an urging member 62. The bracket is mountable to a frame (a mounting lug 24b) of the bicycle. The tension member is secured to the bracket immovably in a first direction and movably in a second direction different from the first direction, and can apply tension to the chain. The securing position adjustment mechanism can adjust a securing position of the tension member to the bracket in the first direction by sliding. The urging member urges the tension member in the second direction.