Mr Sanguan said he left the country on May 22, when the National Council for Peace and Order (NCPO) took over the country's administration.
He learned that he had been summoned to report to the NCPO on June 10, but he was at the time not able to do so as he was still abroad, and did not think about surrendering because he believed he had not done anything wrong.
The former MP said when the arrest warrant was issued on June 16 for his arrest he decided to return home.
He arrived back on the morning of June 25 and went to see a lawyer before giving himself up to the police.
Pol Col Prasopchoke Prommoon, a deputy CSD chief, said Mr Sanguan would be formally charged and held for questioning. Police would ask the military to deliver him to the military court on Thursday to seek and would seek approval for his further detention.