In the second round of voting, in which candidates who passed the first round were required to cast a vote each to each professional group they did not belong to, Somchai won 10 votes.
The two top vote earners in each of 20 professional groups would move to the national level stage of the election.
Pol Lt-General Chatree Boonthawee was second in the same group with Somchai. Chatree got five votes.
Somchai is a brother-in-law of former prime minister Thaksin Shinawatra, the patriarch of the ruling Pheu Thai Party.