To provide a standardized coupler base that has a proper mass minimized due to a small load and that enables conformation of strength when an anticipated load is larger.SOLUTION: A coupler base (10) for a fifth wheel coupler (20) comprises a flat support plane that is provided on a top surface, and two approach angle parts (15) for defining an entrance opening (14), which are provided at a back end (13). The entrance opening (14) is extended to a longitudinal axis (x) of the coupler base (10) and ended in a king pin support area (14a). On an under surface (16) of the coupler base (10), storage structures (17) for a support block are formed on both the sides of the king pin support area (14a) and at a mutual interval on a lateral axis (y), respectively.SELECTED DRAWING: Figure 2
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