A seat belt buckle device that fixes a tongue plate provided at a seat belt, comprising an outer case into which the tongue plate is inserted; a latch member that rotates in response to the insertion of the tongue plate into the outer case and latches the tongue plate ; a release button that releases the latching of the tongue plate by the latch member by sliding into the outer case ; and a counterweight that is rotated by a force received from the release button and resists to the sliding of the release button , wherein the counterweight has: a first rotating shaft that causes the counterweight to rotate with respect to the outer case ; and a second rotating shaft that is engaged with a bearing groove formed in the release button and receives the force that rotates the counterweight due to the sliding of the release button , and the second rotating shaft has a portion, in which part of an outer circumferential surface of said second rotating shaft is missing when compared with said second rotating shaft having a round cross section, with this portion being configured to come into contact with the bearing groove of the release button when the release button slides into the outer case , characterized in that the second rotating shaft of the counterweight comes into contact with the bearing groove of the release button by an outer circumferential surface except the portion, in which part of an outer circumferential surface of said second rotating shaft is missing when compared with said second rotating shaft having a round cross section when the seat belt buckle device fixes the tongue plate ; and comes into contact with the bearing groove of the release button by the portion, in which part of an outer circumferential surface of said second rotating shaft is missing when compared with said second rotating shaft having a round cross section, when a sliding distance of the release button into the outer case is the largest. a first rotating shaft that causes the counterweight to rotate with respect to the outer case ; and a second rotating shaft that is engaged with a bearing groove formed in the release button and receives the force that rotates the counterweight due to the sliding of the release button , and the second rotating shaft has a portion, in which part of an outer circumferential surface of said second rotating shaft is missing when compared with said second rotating shaft having a round cross section, with this portion being configured to come into contact with the bearing groove of the release button when the release button slides into the outer case , characterized in that the second rotating shaft of the counterweight comes into contact with the bearing groove of the release button by an outer circumferential surface except the portion, in which part of an outer circumferential surface of said second rotating shaft is missing when compared with said second rotating shaft having a round cross section when the seat belt buckle device fixes the tongue plate ; and comes into contact with the bearing groove of the release button by the portion, in which part of an outer circumferential surface of said second rotating shaft is missing when compared with said second rotating shaft having a round cross section, when a sliding distance of the release button into the outer case is the largest. <EOS>