Abstract: Disclosed is a rotation transmission mechanism including: an internal rotation member inserted by a crankshaft of a bicycle; and an external rotation member configured on the internal rotation member. The internal rotation member includes: an internal rotation member body. The external rotation member includes: a circular ring part configured at an outer side of the outer circumferential convex parts; and inner circumferential convex parts protruding to an inner circumferential side of the circular ring part and configured alternately with the outer circumferential convex parts. Elastic deformation parts are configured between the outer circumferential convex parts and the inner circumferential convex parts on a rotation direction side of the advancing outer circumferential convex parts. The outer circumferential convex parts are formed in such a manner that a surface on the rotation direction side is greater than a surface on an opposite side of the rotation direction.