A buckle device capable of reducing an impact noise generated when a tongue plate (2) is inserted and eliminating a coil spring member energizing a release button (50) and a lock pin holder so as to simplify a structure, wherein a lock bar (30) has both end parts inserted into slots (16) and is mounted on a pair of side plate parts (12), both end parts of the lock bar (30) are engaged with a pair of guide grooves (54) in a pair of guide wall parts (52) of a synthetic resin release button (50) and, when the tongue plate (2) is inserted, the lock bar (30) is moved rapidly to the front end part of the slots (16) and, in this case, the end parts of the lock bar (30) are supported on a supporting part (55) at the end of the guide groove (54), not on the end of the slots (16).