Police searched Prakasit and the driver of the bike after they noticed them “behaving suspiciously” at a police checkpoint at the Sakoo-Cherng Talay.
They found and seized the pen gun, a .38-calibre bullet and a pocket knife from Prakasit.
Prakasit admitted that all the item belonged to him, saying that he carried them for his own protection.
Unimpressed, police charged him with illegal possession of firearm and carrying a gun in a public area without a permit.
Pen guns are small weapons carrying a single bullet and, as the name suggests, disguised to look like a pen. In Thailand, many are homemade.