The blast occurred on the third anniversary of the junta's 22 May coup and struck a VIP waiting room inside a military-owned hospital in Bangkok, wounding 21 people.
Deputy junta leader Prawit Wongsuwan told reporters today that an unnamed 62-year-old man had been detained for carrying out the blast.
"He did it absolutely," Prawit, a retired general, told reporters, adding the man "hated the military".
It is not clear when the man was arrested. The junta has granted itself the power to detain anyone incommunicado for seven days but suspects are frequently held for longer periods before authorities announce their arrest.
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Despite a veneer of stability Thais remain divided and uncertain over the future three years after the ousting of the elected government of Yingluck Shinawatra.
Over the past 10 years Thais have witnessed repeated rounds of deadly protests, a string of short-lived governments and two military coups that deposed elected leaders.
The junta says its 2014 coup -- the 12th time generals have successfully seized power -- was needed to bring stability and root out corruption.
But critics say the military is deeply hostile to ousted premiers Thaksin Shinawatra and his sister Yingluck, whose parties have won every poll since 2001.
Prawit declined to say whether the arrested man belonged to any political faction.
Local media reported he had pictures at his home of Thaksin, who lives in self exile overseas.
"I am confident that he (Thaksin) does not back violence and the Pheu Thai party has already condemned (this attack)," she told reporters.
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