Mr Aree said the inmate, identified as Anukoon Pasookf, had been sentenced to life in prison on drug offence and was fighting his case in the Appeal Court.
An initial post-mortem examination found Anukoon, 33, hid 20 condoms containing a combined 165 grammes of ice drugs with an estimated street value of 8-10 million baht. The death was believed to have been caused by the bursting of some of the condoms in his stomach, Mr Aree said.
After searching his cell, authorities found another condom with crystal meth thought to have been discharged from his body.
Mr Aree said Anukoon was taken to testify before the Saraburi Provincial Court on Wednesday and prison guards performed regular security checks on him but found no irregularities before escorting him back to his cell.
Officials closely monitored him the next day after Anukoon did not come out from his cell to eat nor join other inmates in general activities. The inmate was later found dead in his cell on Friday morning.
Mr Aree believed Anukoon swallowed the drug while being taken to court on Tuesday and authorities were investigating further how it happened.
Mr Aree said an x-ray machine would be installed at the Klongpai Central Prison in August to detect illegal items in the inmates' body.
An inmate was recently caught hiding three mobile phones up his rectum. He finally confessed to guards as he suffered severe stomach pains and asked for urgent medical aid, the prison commander said. The minimum selling price of a smuggled mobile phone in prison was 800,000 baht.