The accident occurred about 12.30am on Monday. The bus was on its way from Khon Kaen to Chiang Mai.
Ban Yaeng police station sent a rescue team to the scene who took injured passengers to hospitals in the area.
A passenger on his way to Phitsanulok said the bus had been travelling downhill to the bridge when the driver suddenly braked, causing the vehicle to lose balance, hit the bridge rail and plunge to the ground about five metres below road level. The passenger was slightly injured.