The ruling went to Mahamasakree Mahama-useng, also known as Ahama Ma-useng, whom public prosecutors had accused of illegally possessing and carrying firearms and being involved in the separatist movement in Pattani, Yala and Narathiwat provinces from June to August 2002.
The court found that public prosecutors had only oral evidence based on accounts of four police detectives and interrogators and the defendant's stepfather who had testified as long as two years after the alleged wrongdoing.
Besides, the stepfather - who might not know Thai - had signed his Thai written account without anyone translating the content for him. The defendant had argued that the stepfather had stolen his motorcycle and had called authorities to search his house for illegal items but nothing had not been found. In addition, there was not an eyewitness in the case.
The court acquitted Mr Mahamasakree but extended his detention pending an appeal.
The defendant's elder sister was at the court on Thursday and cried happily for the ruling. However, she did not bring an asset to seek his release on bail while his lawyer was working in Nakhon Pathom province.
Police arrested Mr Mahamasakree in Phuket on Sept 21, 2012, and he has been detained during investigation.