An extreme ultraviolet light source apparatus includes a main body including a supply section to which an extreme ultraviolet radiation seed is supplied, and an emission part configured to emit extreme ultraviolet, an excitation unit provided in the main body and configured to generate a plasma by exciting the extreme ultraviolet radiation seed, an optical condensing unit provided in the main body and configured to converge extreme ultraviolet, which is radiated from the plasma, at the emission part, a trap provided between the excitation unit and the optical condensing unit, a first positioning mechanism connected to the trap and configured to adjust at least one of a position and an angle of the trap, and a measuring unit configured to measure a far field distribution image of the plasma on the basis of the extreme ultraviolet which is emitted from the emission part, thereby to operate the first positioning mechanism.