Crowd of locals and tourists were gathering on the 500-metre long and two metre wide bamboo bridge to offer alms to about 30 monks when it suddenly collapsed due to overweight.
A dozen of people were injured, two seriously when they were trapped under the concrete polls supporting the bridge.
They were rushed to Mae Hong Son hospital for treatment of bruises and broken legs.
The merit-making ceremony was chaired by Mae Hong Son deputy governor Virun Pantevi.
He said the bridge was recently damaged by floods and has just been repaired in August.
Overloading of people was blamed as a cause of the collapse.
Su Tong Pae bridge is the longest bamboo bridge in the country built by local people of Kung Mai Sak in Mae Hong Son, to facilitate monks of Wat Phu Sa Ma temple on their morning alms round.
The bridge runs for 500 metres across rice fields and Mae Sa Nga river linking Kung Mai Sak village in Muang district and Suantham Pusama meditation centre. It is eight kilometres north of Mae Hong Son town.
It is one of the unseen tourist attractions in Mae Hong Son.