Complex sentence: No law forbids a foreign-born Japanese to be elected as a member of Diet (as Marutei Tsurunen in fact became one), thus in theory they may become the Prime Minister of Japan.
Split into simpler sentences: No law forbids a foreign-born Japanese to be elected as a member of Diet (as Marutei Tsurunen in fact became one). Theoretically, therefore, a foreign-born Japanese can become the Prime Minister of Japan.