A seat belt device includes a spindle (11) for retracting a seat belt (13), an electric actuator (55), a pre-tensioner (14) and a power transmission mechanism (20), wherein the power transmission mechanism (20) can reversibly switch the connection and disconnection between the electric actuator (55) and the spindle (11) before the pre-tensioner (14) is activated, while when the pre-tensioner (14) is activated. The power transmission mechanism (20) non-reversibly interrupts the connection between the electric actuator (55) and the spindle (11) via an elastic piece (60) when a torque difference between an actuator-side gear (80) and a spindle-side gear (82) becomes larger than a predetermined value.