Abstract: A buckle device into which a tongue plate provided on a seat belt of a vehicle is to be inserted, and which detachably engages with the inserted tongue plate. The buckle device includes a hollow buckle body, an operating member configured to be operated in response to a release operation for releasing engagement between the tongue plate and the buckle device, the operating member being supported, in the buckle body, to be slidable in an insertion direction of the tongue plate, and an inertia member supported, in the buckle body, to be rotatable about a rotation center axis extending in a width direction of the tongue plate.