A seat belt locking buckle configured to allow a seat belt locking tongue to be stably and firmly latched and reduce the thickness of a seat belt locking buckle. When a tongue latch member is inserted into a seat belt locking buckle, an ejector is moved. Thus, a buckle latch member 13 becomes rotatable. Consequently, a slider moves along a plane that is parallel or substantially parallel to a plane of the motion of the tongue latch member and pushes a first latch guide portion of the buckle latch member. Thus, the buckle latch member is rotated from a non-latch position to a latch position. In this manner, the tongue latch member is latched by the buckle latch member.