PURPOSE: To easily unlock a buckle, by providing an automatic buckle unlocking device and automatically unlocking the buckle through said unlocking device so as to release a seat belt from its locked state.

CONSTITUTION: When the crew get off from a vehicle, if a key switch is turned off, an electric current is allowed to flow in a solenoid 2 for a certair length of time. In consequence, the solenoid 2 is excited, and its electromagnetic force attracts an operation shaft 21 in the right direction. In this way, a slide button 12 is also moved in the right direction, and a lever 13 is depressed by a slope face 12a formed in one end of the slide button 12. A spring 14 is comparessed flat while a protrusion 13a of the lever 13 is disengaged from a hole 6a of a tongue plate 6. Accordingly, a seat belt 8 is taken up together with the tongue plate 6 by a belt reel device, and the crew is released from his locked condition by the seat belt 8.