A tongue assembly for use in a seat belt device is formed of a tongue main body engaging a buckle and a slider slidably disposed over the main body. A first slit is situated in a middle portion of the main body to extend in a direction perpendicular to an inserting direction of the main body relative to the buckle. A first wall portion is fixed to the main body to project upwardly from an upper surface of the main body along the first slit at a side of a rear portion. A second wall portion is formed in an upper side of the slider and has a surface opposing to the first wall portion. A second slit is formed at a lower side of the slider so that a rear edge of the second slit at a side of a rear portion of the main body is situated close to an insertion portion of the main body with respect to the surface of the second wall. A third slit is further formed in the rear portion of the main body. A seat belt extends between the first and second wall portions, through the first and second slits, under the lower side of the slider and through the third slit so that when a tension is applied from the side of the third slit, the slider is pulled toward the rear portion of the main body to urge the second wall portion of the slider to the first wall portion to thereby strongly hold the seat belt between the first and second wall portions.