A seatbelt retractor is provided that can be formed in a smaller size yet properly operated, and capable of smoothly providing seatbelt tension, and a seatbelt assembly including such a seatbelt retractor. The seatbelt retractor includes a frame, a seatbelt, a spool that takes up the seatbelt, the spool being rotatably supported by the frame, a lock mechanism that allows the spool to rotate in a non-activated state and restricts, upon being activated, the spool from rotating in the seatbelt withdrawal direction, and an energy absorption mechanism that limits the load applied to the seatbelt to thereby absorb and alleviate energy of the occupant, and the energy absorption mechanism includes a first torsion bar concentrically enclosed in the spool with one end portion retained by the spool and the other end portion disposed to be locked by the lock mechanism, and disposed to be rotated by torsional force, a second torsion bar eccentrically enclosed in the spool with one end portion 24a disposed to be locked by the lock mechanism, and disposed to be rotated by torsional force, and a moving member that rotates interlocked with the spool, and moves between the first position for retaining the other end portion of the second torsion bar and the second position spaced from the other end portion of the second torsion bar.