A Sing Buri man was found dead inside his car on a roadside in Ang Thong's Muang district early on Thursday.
Pol Senior Sgt Maj Muangman Jinyai from the Muang Ang Thong police found the body at 1.30am while he was patrolling the Ang Thong-Ayutthaya road in Moo 8 village, Tambon Phosa.
Muangman said he spotted the car parked but its engine was still running so he went to check and saw the driver, who appeared to be sleeping.
He called out to the driver, who failed to reply so he called a local rescue foundation, whose officials checked and found that he was dead.
The man was identified as Thani Srinart, 39, from Sing Buri's Tha Chang district.
Police suspected that Thani parked his car after being overwhelmed by sickness and died.
The body was sent for a post-mortem at Thammasat University hospital.