A seat belt retractor of a seat belt apparatus to be installed in a vehicle is structured such that when a vehicle door corresponding to a seat belt moves from the closed state to the open state, the seat belt retractor is configured to be in the power transmission operation mode so as to perform the seat belt winding-up action and, after the seat belt winding-up action, the seat belt retractor is configured to be in the power transmission allowing mode so as to maintain the mechanically connected state between a spool and a power transmission mechanism.