In a belt buckle device including a tongue piece (75, 76; 210; 310) connected to an end of a belt extending in a longitudinal direction and a hook member (68, 70; 220; 320) configured to be selectively connected to the tongue piece by engaging a first lateral bar of the tongue piece in a hook opening (111; 226; 326) defined by a hook portion (71, 72; 225; 324) thereof, a longitudinal dimension (B) between the free end of the tongue piece and a base end side of the first lateral bar is substantially equal to or smaller than a width (A) of the entrance of the hook opening, and the effective thickness (C) of the first lateral bar is greater than the width (A) of the entrance of the hook opening. Thus, the first lateral bar is held inside the hook opening as long as the tongue piece is kept in a flat position in a reliable manner. In particular, the first lateral bar can be freely introduced into the hook opening simply by raising the base end of the tongue piece while the first lateral bar is applied to the entrance of the hook opening so that the belt buckle device can be readily fastened even when the belt buckle device cannot be viewed from a proper viewing angle. Also, the belt buckle device can be readily fastened with a minimum dexterity requirement and very quickly.