CIB commissioner Pol Lt Gen Pongpat Chayaphan said Somchai was arrested at the Lat Krabang tollgate while travelling in a black Lexus SUV. Police found six bullets in the vehicle, but no firearms, he added.
Somchai's vehicle was on the outbound lane of the motorway during the arrest. He was travelling with a woman and a man, believed to be his driver.
Somchai was leaving Bangkok after a visit to Samitivej Hospital. Police found a medical bill under the name of Kim Sae Tung.
He was taken to the Crime Suppression Division's office in Chokchai for interrogation, Pol Lt Gen Pongpat said.
The Supreme Court on March 12 last year sentenced Somchai in absentia to 25 years in prison for hiring a gunman to murder Prayoon Sitthichoke, alias Kamnan Yoon, the head of tambon Samet in Muang district of Chon Buri, at a wedding in March 2003.
On June 21, 2004, the Criminal Court sentenced him and Passakorn Homhual, a former member of the Saen Suk municipality council in Chon Buri province, for the same offence. The Appeals Court on Oct 12, 2006 upheld the judgement.
Somchai was earlier found guilty by the Chon Buri Court of graft over the purchase of a 140-rai block of land valued at 93 million baht in Bang Lamung district, Chon Buri, by Pattaya City between 1992 and 1993 for use as a rubbish landfill, and sentenced to five years and four months in jail.
Somchai was earlier reported to have fled to Cambodia.